First, you need to make sure that Titan has enough gravity for your race. If you've got a colony cost that isn't "N/A" then you're ok. There are 5 things that have to happen to get colony cost to zero IIRC:
1) No poisonous gases.
2) Enough oxygen in atmosphere (controlled by race characteristics)
3) Percent oxygen less than 30%
4) Pressure not too high
5) Temperature in right range. (controlled by race characteristics)
The first three have "threshold" cost - it goes from something like 2.0 to zero the moment the threshold is crossed. In contrast, temperature and pressure are "gradual" costs - the further you are from your race's allowed range, the more it costs.
The other thing to realize is that the costs are NOT additive - instead the worst cost is the colonization cost. So what you want to do is work on the worst one first. For colony cost >2, I usually work on temperature and/or pressure first until the cost reaches two, then work on oxygen until it crosses the threshold, then go back to fixing temperature and/or pressure. For cold worlds with thin atmosphere, the first and last steps usually involve pumping in greenhouse gasses, which help both to bring the temp up and get the %O2 down. A lot of the time the temp is in a pretty good range by the time the O2 is correct and below 30%.
Waresky already mentioned how to work the controls, so I won't repeat that....
John
