“I think I may have a decent chance of winning it.”
You've entered an art competition. You feel quite confident about your submission. Now you're talking with your girlfriend, you say this about whether you'll win or not.
I think I may have a decent chance of winning it.
When you "have a chance of winning" something, it means that you might win it. In a contest, if you think that you are better or as good as the other contestants, you can say that you "have a chance".
This phrase is similar to "have a shot at (doing something)" One difference is that you mostly use "have a shot" in positive situations, but you can use "have a chance" in both positive and negative circumstances. For example:
These guys are pros. I don't have a chance against them.
"Decent" means "pretty good" or "not bad". Use it when something isn't very good, but isn't bad either:
A: How was the food?
B: It was decent.
The word "may" can be used in the same way as "will" to talk about the future, but you use it when you're not quite sure. Its meaning is similar to "maybe", but you use it before a verb like "will":
I may try to meet up with Heidi while I'm in Chicago.
It may rain later today.
"May" has other uses as well, like making guesses about the past:
I may not have installed it correctly.