Get yourself some skratch records. Goto Juno/Chemical Records (Tho I picked one from HMV last week ) & get dedicated skratch plates. It will save you f*cking up/marking good tunes. Pratice, pratice & pratice some more. It will come, it just takes time.
there's a scratch tutorial series by dj iq on dnb arena which will show you the basics and isn't too bad, and there's the qbert diy dvd's which are really good to learn from, apart from that, like the others said, practise, practise, practise.