"Go for it" can mean "Do whatever you want to, I don't care."
Fred: "I was thinking of asking that cute girl in the Accounting Department out for lunch. What do you think?"
John: "Sure, go for it."
It can also mean "I don't think you'll succeed, but try it if you want to."
Brenda: "Are you going away this weekend?"
Kelly: "Yes. I'm leaving right from work Friday at 5:00, then going straight to the airport. My flight leaves at 6:30."
Brenda: "You'll never make it in time!"
Kelly: "I'll be fine. I know a shortcut, and I'm a fast driver."
Brenda: "I would have booked a later flight, but if you think you can make it all the way out to the airport in Friday rush hour traffic in less than an hour, then go for it."
It can also mean "Go ahead! I think you've got a very good chance of succeeding! Do your best!"
James: "Did you see that notice on the bulletin board? They're going to accept applications for a new executive manager for the IT department."
Melissa: "You should apply for that position. You've been with the company for 10 years, and you know that job inside out."
James: "I don't know...do you really think I stand a chance?"
Melissa: "Absolutely! No one here knows the computer system like you do. Go for it!"