Been playing guitar for a couple of weeks, and want to know the best ways to play chords, mainly barre chords (eg. F). Anybody know some good resources?

Take F for example, i was told to use index on the E string, then 3rd and 4th fingers on the next two strings and middle finger on the G. Is this right?

Someone else told me to cover the A and D strings with my 3rd finger, but i cant quite manage it. Is it just a matter of practice?

Lots of practice. Barre chords are the toughest thing to start up on. Keep at it, and you'll get it.
Here's what I do for 6-string major barre chords: lay your finger across all 6 strings (it'll take practice to get it so all the notes ring out), use your ring finger and your pinky to fret the 4th and 5th strings 2 frets up, and use your middle finger on the 3rd string one fret up.

Then just practice more after that.