It all depends on the shop, and what you bet on.
Different shops offer you different odds. You will hear odds such as '2 to 1' thrown around a lot. This basically means there is a 1 in 2 chance of what your betting on actually happening.
You can also bet on a variety of different factors when it comes to football. For example, you can bet that one team will win, or you can bet that a certain score will happen, you can even bet on the amount of corners that happen.
Bookmakers do not take commission, meaning they will not take a % of your winnings for using their service. However, the odds are generally in their favour, meaning you have a slightly better chance to LOSE money than you do to win. This is what keeps bookmakers in business.
There are so many different factors. You should start off by going to your local betting shop and betting a small amount (A few dollars/pounds/whatever) on a bet like Man Utd to win against chelsea, which will probably be around 2 to 1.
Good luck with your gambling adventures.