Mostly, you will have to learn to make the things you miss at home. It's really not that hard, and it often tastes better when it's homemade, anyway. As far as real American junk food and things go, you can order those things online, if you are desperate, or check out the threads on the forum regarding where to find American foods here. It can be expensive, but probably worth it if there are a few things you are finding it hard to live without and would really help your homesickness. Otherwise, try to find things here that you like. There are lots of foods here that aren't like anything in the US, which can be a very good thing, too. Be adventurous!
Oh, and if you need cheese for tacos, it's often best to buy the cheapest coloured mild cheddar you can find. It tastes very similar to the cheddar in the US. Steer clear of the extra mature cheddar for Mexican food! It's definitely no bueno in enchiladas and things like that. One brand of cheese that I really like a lot is called Wexford's, I think. It's a really mild Irish cheddar that is great for Mexican food and things like that, surprisingly.
They have it at our local Sainsburys, so you might find it there. Otherwise, just gorge on things they do have here, like Ritz crackers, Oreos, and Doritos (the tangy cheese are the same as nacho, and the cool original is the same as ranch!)