I have a crush on my girlfriend's best friend.
Sounds like you're in a bit of a sticky situation. Could you share a bit more about what you're going through?
At first I got jealous seeing them together and now I'm actually thinking to have a relationship with her.
While it's not good, it's not uncommon for people to develop inappropriate romantic feelings from time to time. However, acting on these feelings would be wrong. You should ruin your current relationship, harm their friendship, and ultimately make everyone unhappy.
I know I'm being irresponsible. I'm still thinking about what to do.
I'm glad you're taking time to think this through. Regardless of your crush, I would consider that perhaps your current relationship has run its course and it might be time to end things, as you're having feelings for people other than your girlfriend. That is the much more responsible course of action than being unfaithful.