Don't try to start out being a father...begin slowly with "Big Brother" type activities. Start with short visits and activities. Depending on his age; if he is 4 or less, maybe you could offer to babysit him. 5-7 maybe walk him to school or keep him after school (once or twice a week) and help with his homework. 8-12...go fishing, go-carting, paint ball then lunch or dinner. 13-16 teach him some skills like working on a car, carpentry, bbqing or whatever talents you have.
Depending on how old your child is Start easing your way into their life. If you go full throttle your child may not be receptive and wonder why all of a sudden you are so involved out of no where. Build a fatherly friendship ( don't let him get away with things, but be friendly and caring.) keep the mother involved and express your true intent and ask her help you. Express you want to be a father.
I feel ya' man. I was apart from my son from when he was 12 until he was 20. Now, he is 25, we live and work together. It is very difficult sometimes. I missed out on being there for some of the most important and formative years of his live.
We try to talk things out calmly. We go to a councilor once a week, and that seems to help a lot. We love each other, but having been apart for all that time, we did not grow together through some very important times of his life. We try very hard, but sometimes we have heated arguments.
We have found out that rather than resolve or come to a conclusion in the arguments, we can just leave them, and go on to something else. Sometimes I feel like this is cheating, but at the same time it seems to work, and things go better for us.
I think if you truly want to be close, it will be a long and difficult journey for you both. You both have to want it, and as the older, more mature of the two, sometimes you will just have to suck it up and be willing to be wrong.
Your son will always be one of the closest persons to you in your entire life. It is a blessing to have him in your life. Never give up.