Tips for beginners trying to improve their game in War Thunder?
When I first started playing, I too had quite a difficult time picking up the game's nuances, and could often go through several games without picking up a single kill. Now I've managed to find my place a bit more, and often end up finishing a match with a respectable score. There is mounds of advice you can give, but based on my experiences, here are just a few more important tips I'd give for improving your early game in War Thunder:
In no particular order:
Watch other players: As mentioned in Neffer_23's comment, watching one or two YouTuber's tutorials on the game before going into a match can really help you out in the beginning. I personally haven't seen the ones mentioned in said comment, but there are plenty of streamers out there with plenty of content you can take advantage of. In addition, the War Thunder team have released some tutorials in a series called Barrel Roll, which gives one or two more advanced tips, including some plane-specific ones. Be careful with the plane specific tips though! (see point 4)
Advance from tutorials: Just watching streamers playing the game for fun can give an insight on how to improve your game as you progress beyond what the tutorials can offer. If possible, try to watch a video with a plane similar to your preferred one, (see point 4) as strategies can vary quite significantly between models.
Pick the right nation: It's important that you're playing a nation that suits your play style, and unfortunately, the game gives minimal information when choosing a country to play. While all the biplanes are very similar in offensive capability and handling, the second you begin progressing up that tree, you'll easily spot the differences. If you're after hit and run tactics (Boom & Zoom), then Germany would be your preferred tree. If you want to get in close to your opponent and out-maneuver him before getting a shot on his back(Turn-fighting), then the Japanese tree would be perfect. It's worth taking half an hour or so to run through the research trees, hovering your mouse over each plane and reading it's stats, such as top speed, turn time, and armament information. If possible, test-fly as many planes as you can in single player games before taking them out onto the PvP battlefield for real.
Use the right strategy for the plane you're flying: Not all planes are the same, and certain ones are better suited to certain roles. Just because a bomber has a front facing armament like the B7A2 Ryusei, it doesn't mean it should be used as a fighter. Go in for a head on shoot-off with a plane from the A6M series and you'll be shot down very quickly due to the fragile nature of the plane. Try and turn-fight in something like a Beaufighter, then you'll be quickly out-turned and shot down.
Don't be afraid to try something new: There's a lot of diversity in War Thunder, so even if you've got a strategy that's working, don't feel anxious about switching it up for a match or two. While I personally play the Japanese and British trees in a highly turn-fighting based form of combat, I do occasionally slip over to the German tree for a different experience.
Allocate your crew points: After a few matches, you'll notice that each vehicle slot will have a little white face over it. A right click on the slot will allow you to bring up the "Crew" menu, where you can allocate points you earn in games. Be careful! Points apply to the slot, and carry over between aircraft. Therefore, make sure you spend carefully - have specific slots for one man fighters, where you focus all the points on the pilot and ground crew, and set one aside for bombers where a larger portion of your points can be put into the gunner positions. Having a BF 109's crew with 50 hard-earned points in gunner accuracy will get you nothing.
Use the aiming reticle properly: If you're a new player, chances are you'll be spending a lot of time in Arcade mode, which means you'll have access to the aiming reticle, and that guy can be your best friend if used properly. Before spawning in, you'll notice one of your settings reads something along the lines of "aiming distance", and this defaults to 400m. When chasing an opponent, the red text under their name will tell you how far away they are, meaning that when the text reads 0.4km, placing your gun sights right on top of that reticle means your shots should be spot on. If they're further away, or closer to you, adjust accordingly, and you'll start seeing a lot more shots find their mark.
Use the right flaps for the occasion: Take-off and landing are pretty obvious, combat flaps (not on every plane) are covered in this post here.
Don't use Stealth ammo straight away: Stealth ammo does what it says on the tin - it doesn't give away your position as easily by removing tracer rounds from your ammunition, meaning the enemy can't see the direction your shots are coming from. However, this works both ways - you can't see where your shots are going. Until you've picked up the ability to successfully predict where your bullets will fly to, use an easy to spot ammunition type, such as the Tracer variety.
Patience is a virtue! Don't fire your guns the second you see an enemy, be patient. While most weapons have the potential to hit a target 800+m away, you're going to have a hell of a hard job doing it, and alert the enemy to your position in the meantime. Once you unlock cannons, ammo isn't plentiful, and you're going to want each shot of your 200 round clip to find its mark. Therefore, wait until you're confident to fire, and let the gap between you close. Even with an enemy bearing down on your back, don't rush your shots - it won't pay off.
Don't be the first guy in: As much as I hate to see people doing loops to avoid being the first person to meet the enemy team, as a new player, it's understandable if you want to have a backseat view at first. Wait for one or two skirmishes to break out, and then try and attack somebody on the tail of an ally. They'll be preoccupied with shooting him down, and this often gives you a nice opportunity to score a kill. Your team-mate will be pleased with you bailing him out, as well.
Stick with your allies: As much as you love being a lone wolf, don't stray too far from your team. While you might end up with more assists than kills, it's better than chasing an enemy into the heart of his team, where you'll be way out of your depth in a 1v5 scenario and finishing with a score of 0-Kills 4-Deaths 0-Assists. You won't have fun being shot down repeatedly, and your team won't be pleased with you either.
Don't worry about getting the Terror of the Sky medal each game: It's not a bad thing that you're focusing on objectives. A lot of the time, a game can end quickly and not in your favour because everyone else on your team is swarming around one or two enemy planes while their bombers wreak havoc behind your lines. You still get points and experience for taking out non-player targets, so don't feel like your contribution is any less. I for one always keep at least one bomber in all my loadouts, so I can help my team in a different way if it calls for it.
Stick with Arcade mode at first: While Realistic and Simulator battles do have enticing bonuses, don't be tempted to join in until you've mastered the basics. Realistic has no aiming reticle to help you out, Simulator sticks you in first person and removes both "mouse-aim" and enemy tags from the map, while both have only a single spawn, realistic flight mechanics, and a long flight from take-off to meet the enemy. Arcade is invaluable for new players with it's multiple respawns, forgiving handling and for the sheer speed you can get back into the action.
If you want to know a specific thing in a match, ask a player - the community is by no means the worst one in terms of MMO's.
There are other things I could put, but it's easier to pick most things up as you play the game, and that also helps you develop your own style better as well. Learn from your mistakes, and before long, you'll be a top pilot!