The best advice is the most obvious - stuff like "don't put all your eggs in one basket", "don't invest more than you are prepared to lose", etc. Boncey suggests iii, and you can register for free and set up portfolios and so on. I suggest you "paper trade" for a while, i.e., set up a portfolio of a few shares you fancy (without actually buying them!), see how it goes, see it your judgement appears sound. If it does, give it a whirl! But many people have doubts about where the market is heading. It's not an easy time to be starting out. Finally, iii have lots of bulletin boards about specific shares. These can be useful, but be very wary of "hot tips" and so forth - there are lots of sharks, liars, and plain stupid people out there (this is not iii's fault, obviously!)
Good luck!