How do I fight multiple enemies surrounding me?
If you're struggling with groups of enemies then there are a number of options:
Don't be afraid to ignore the combat completely. There are very few encounters that are actually necessary, most can be avoided by sneaking around them or taking an alternative path. Worst case scenario just run straight through them, monsters only tend to follow for a short time, particularly if you can break line of sight.
Pick them off one by one if possible. This can be tricky as many groups will alert other members, so you have to kill them quickly. Bombs and arrows can both do a lot of damage with out actually making enemies aware of your presence. I have killed many enemies by raining bombs down on them from a height (be aware though that even non bow armed enemies have ranged attacks and will throw rocks at you if they do spot you)
Edit: As mentioned by Batophobia in the comments you should also make sure to use the environment to good effect. Puddles of water can be exploited with Lightning Arrows, grass can be set fire to, boulders can be pushed on to enemies and metal objects can be swung around.SGR's tip is a good one for group combat. Get a good two handed weapon and start charging an attack as the enemies run towards you, if you time it right you should be swinging your weapon around wildly by the time they reach you and you can decimate a group pretty quickly.
Alternatively get a good spear and back away while using the spears reach to take out the closest enemies until the group has been thinned a little.
Always watch out for archers. Two or more archers plus some melee enemies can cause you a lot of trouble. If you can sneak in and kill any archers first (or make a beeline for them and take them out first) this can really improve your chances.
Do some more Shrines and get some more heart containers. Try and find some better weapons or improve your armour. I struggled a bit with combat at first but with better armour and weapons and a little practice I rarely get in trouble at all.
Make use of cooking to create food and elixirs. You can always eat from the menu, so if you are reduced to low health you can eat to heal yourself (as long as you have prepared items, some raw foods heal but not as much as cooked foods). You can also cook food to provide many different bonuses, like extra attack or defence. As Batophobia mentioned in the comments a bonus hearts meal is particularly useful because it also fully heals you.
One last ditch method, provided you can buy enough time to pull it off, is to use the special attack of a Two-handed weapon (not a spear).
Simply hold down the attack button while having a two handed weapon equipped and Link will start spinning. Two handed weapons have a greater chance of knocking an enemy back or otherwise stunning them, and the special attack causes you to attack all 360 degrees around you. This should hopefully knock down a couple of smaller enemies and stagger bigger ones, allowing you to get away.
If you encounter a bigger group of bad guys it is usually a good idea to keep them all the time in front of you. You can do so by focussing one and jump backwards until everyone is in front of you. Then you could use a weapon with higher range (like a spear or a two-handed sword) an attack them.
But keep in mind: Are you already prepared for those enemies? Do you have enough hearts? Is your armour high and your weapon strong enough?
It is also a good idea to use a shield. It is also easier if you already unlocked Daruk's Protection. Maybe you should practice the counter-attack?
You could also use some of your bombs. Try to get some distance to the group and throw one. This separates them and knocks them down.