Lots of good advice in this thread, especially the first couple pages. I'd like to emphasize one big thing:
Young people - in your 20s - save NOW. Compound interest is the best investment you will ever make. The other questions like 401k vs Roth, which stock fund, etc -- those are important, but time trumps them.
Seriously, if you put $5k per year away from age 22-30, and make a reasonable 8%, it turns into over $1M.
I think of it this way - because of the time factor multiple:
Spending $1 in your 20s is like spending $20.
Spending $1 in your 30s is like spending $10.
Spending $1 in your 40s is more like spending $5.
Spending $1 in your 50s is like spending $2.
I saved a lot in my 20s and 30s, but will be spending a LOT MORE in my 50s and 60s.