Further to what mandrew has said about this being a deposit bonus:
You now have £50 to bet with and you want to lose this all in one go. Bet the full 50 on something unlikely to happen but remember the higher the odds, the bigger the float you'll need at betfair for your lay bet. Should you win at Bwin you'll have to complete their WR wagering requirements (x5 or £150 of bets) before you can withdraw any winnings. Again try to do this in one go so if you do bust out of the bookie, the bonus is in the bag.
When you're more experienced you'll appreciate the benefits of " underlaying" your lay risk amount by a couple of £s on your first £50 bet in case you happen to win at the bookie at the first attempt ( as you'll have "lost" less at betfair accordingly.
Use the oddsmatcher to find the best football odds you can but beware you may not be able to make "big" bets on obscure markets.
Oh and welcome to FEM