If you have a fair coin (not weighted, double faced, shaved, etc) then we usually assume the chances are 50/50 between heads and tails. However, there are a number of other factors that can change that. There's a 31 page study (!) on the science of coin flipping that has come up with 7 different premises for the physics of coin flipping. For example, a coin that is caught with someone's hand has a 51% chance of landing on the same side it was launched.
The source gives 7 tips to gain an advantage at coin flipping. Here's a few: try to alway toss and choose, if you catch the coin, flip it some times (but not every time) so that people don't take advantage of the principle we described above. Now you know, read the source and you'll be on your way to being 1% more successful at coin tosses