Regarding custody of a child and divorce?

I am filling for divorce from my wife. The problem is that I can't afford a lawyer and honestly, I would just like to make this go as smoothly as possible. The only real issue in all this is custody of my 4 year old son. The current situation is that even though I'm filing, we're in a situation where we have to live together at least until next April. Because of this, I don't know how to go about custody. Her schedule is set, mine has a tendency to fluctuate because of school and work. Mondays are probably my best days to see him but again, it all depends on what my schedule is like for work. Does anyone have any suggestions?