Tritium occurs naturally due to cosmic rays interacting with atmospheric gases. In the most important reaction for natural tritium production, a fast neutron interacts with atmospheric nitrogen
Because of tritium's relatively short half-life, however, tritium produced in this manner does not accumulate over geological timescales, and its natural abundance is negligible. Industrially, tritium is produced in nuclear reactors by neutron activation of lithium-6.
Usage
The emitted electrons from small amounts of Tritium cause phosphors to glow so as to make self-illuminating devices called trasers which are now used in watches and exit signs. It is also used in certain countries to make glowing keychains. In recent years, the same process has been used to make self-illuminating gun sights for firearms
http://en.wikipedia.org/wiki/Tritiumso you don't need fusion (besides industrial fusion doesn't exist yet). I don't know about tritium in Mars atmosphere but I know some think that there could be plenty on the surface of the moon. Which doesn't mean it's easy to extract.
nuclear engines are only at the test stage and right know the biggest launch engine ever made cannot fly. The Space station relies on Soyouz and Ariane. It will take 10 years before we can get back to the moon and it will be a copy of the Apollo mission.
the most promising engines right now are ion propulsion engines but before they can propulse heavy loads at fast speed, a lot of work need to be done. Besides they'll have to be lifted with conventional rockets.