How do I build HUGE constructions in Minecraft?

Rule number one when taking on a big project: Don't do it alone! Recruit some friends to help you out.

You can use a pixel based editor (Photoshop, Paint, etc.) to plan your constructions out. Each pixel can be one block and then you just design what you want.

I don't know of any off the top of my head but scripts can help if you plan to do vrey repetitive actions. Stuff like backing up while placing blocks to create a wall. I'm sure there are macros that would let you capture keyboard/mouse commands which you can use to build your structures.

Copy and Paste. You'll probably want to design the unique elements somewhere (in-game or in an image editing tool), then build them once in-game and copy and paste them in-game using a mod to create the large structures you're looking for.

WorldEdit has copy/paste support but loading it in Single Player requires some careful modifications. It's a bit easier to install if you're working on an SMP server.

For designing your project... Minecraft is really best. Be prepared to build a lot of temporary stairs to stand on as you test out designs. Or install a flying mod.

I would highly recommend the WorldEdit feature.

You could also use the TooManyItems mod for an in game inventory editor and you could install zombe's modpack for the flying ability.

Looking up a few tutorials can't hurt either. Check YouTube.

You can use Paint or Photoshop or indeed any image manipulation program for planning and you could ask a few friends for faster construction.

But in my opinion, MCEdit is the top of the hill when it comes to building giant structures.