Hi,
First port of call should be the forum search. A good many relevant topics have been covered on here, and the information contained is collectively one of the best.
Secondly (though some might argue this more important than the forum) is a copy of the Content Creation Guide for your respective version of Trainz.
Thirdly, when you've decided which program you're going to use (I recommend GMax - search for Turbosquid to find the link) you need to study the various tutorials people have put together. Again, there are links on this forum.
I've not posted the direct links here partly because I'm too lazy to look them up, but mainly because if you can't master the forum search then you will probably struggle with content creation. It's not meant to be a nasty deterrent comment - it's just an opinion that the best creators won't shy away from finding the information themselves.
Finally, the DLS upload is really simple, and full instructions are provided during the process. If you have TRS06 or TC, there is an upload tab underneath the download tab in CMP, or if you have an earlier version you can upload through you Planet Auran profile (you can also do this with TRS06 or TC if desired).
Good luck, and I hope this helps,
BidMod.