You're unlikely to be able to compile a costume that even remotely resembles the pic you've got up there with clothes from your wardrobe, but you might already own items of clothing which you could use for the shirt, trousers, gloves, boots, etc. If you can't be bothered, you could hire a generic soldier costume. Or you could simplify it so you still make a convincing soldier without looking like that specifically. If you want a 'good' costume and you're asking this early, then I'm going to assume that you want to go all the way.
With an outfit like that, you'd definitely want to start early. If you break it down, there are actually heaps of components there. I'd suggest second-hand stores and military surplus stores. Outdoors/camping stores might also have some useful things. Ask friends/relatives if they have anything. Toy guns are usually pretty juvenile looking, so you may have to modify and paint one to suit your needs. You'd need to buy a helmet that already has that basic shape and modify that too, to add all the bits and pieces. (If you were just planning on a simple soldier costume then just ignore this entire latter paragraph.)
