A deposit match bonus, as given by bet365, is probably the best kind of offer from the bookmakers as it instantly increases your betting balance and so your chances of making a profit are increased.
The bookmaker will ask you to enter a bonus code on registration and that will automatically qualify you for a the deposit match bonus. When you make your deposit you'll be credited with bonus funds of the same value and you can use them to place bets.
The advantage of the deposit match bonus funds is that they act like real money. You can use them for any bets, at any odds, and the stakes will be returned with any winnings. This means you can slowly build a bankroll with multiple bets and you don't have to worry about the odds. Once you've reached the wagering requirements the bonus funds and all winnings are turned into real cash that can be withdrawn.