Brilliant post, Branster .
I'd agree with others that it is worth investing in a good foundation - cheap ones can make you look worse than nothing at all IMO. However, it can be very hard to buy the right one - really, you need to take samples home, but most brands don't offer that. My advice would be to go in with a bare face, put some onto your jawline, then check it in a mirror in natural daylight. The one you want will disappear into your natural skintone, and make your skin feel comfortable and soft, not oily or dry. Since you're young, why not try a tinted moisturiser? Most women don't need heavy coverage, and I find a tinted moisturiser (I use Laura Mercier - pricey but SO worth it) is much more flattering. For powder, make sure you go for translucent or yellow-toned: avoid pink-toned powders like the plague (Ruby & Millie is a good range).
Don't think it's worth paying out loads for lipstick or mascara, in my book; unless you fancy a treat. Best mascaras are by Lancome, but Rimmel also very good. Clinique do fantastic lipsticks, but plenty of cheap ranges also very good. Concealer - I find Touche Eclat brings me out in a rash, but many women swear by it. My concealer is from No 17 and it's good; Tesco range also do a great one.
Blush - try a sheer gel or cream rather than a powder. Very flattering.
I agree with all the other messages re. skincare being more important than make-up. Eve Lom is brilliant, but Marks & Spencer do a cheap version in a glass jar that is nearly as good. There are lots of good moisturisers for around £10. Oh, but make sure you get one with a SPF of 15 or above - it really is the best investment you can make in your skin.
I LOVE this thread - could talk about make-up all day!