I assume you are talking about sports betting here otherwise, there is no real way to achieve a profit.
If you really want to win money when gambling, there are two key factors to take into account:
- assessment of the chances of each outcome (probabilities)
- percentage of your bankroll you need to put at stake
The first one is quite easy to do if you have a good knowledge of the sport you bet on and even if you don't, just look at the odds of a few bookmakers to get an idea of the forces in presence.
The second one is more tricky. As far as I know, the best (safe) way is to use a technique called the
Kelly criterion. Depending on a few parameters, it tells you how much (in terms of percentage) you need to bet to achieve the highest possible return in average on the long run.
I also strongly recommend to abuse sure betting or any other forms of arbitrage-like techniques.
The edge, is depending on the game.
If it's a luck game, then you just have to cross fingers.
If it's a skill game, then better practice it a lot before playing for real money, to be the best player.
Then there is risk management, for which the kelly method is pretty solid, although there are other betting systems too.
