To start off..
If you want to model things, or convert things, make peds, players, etc for any GTA.
You need 3ds max, by that. You can either buy it ($4000), download the
trial of it (lasts 30 days) on the autodesk website. Or
Torrent it.You will need a strong computer, but not that strong.
For mapping, i would suggest
MEd, for SA model based maps. Or
KEd for VC/III mapping.
Texture editing, you can use
TXD Workshop, which works for all GTA3, VC, and SA.
For texture editing, i would suggest for a starter. Use
GIMP, then build your way up to
Photoshop. Like i did, GIMP is like "Photoshop's little brother".
Easier, and simpler.
For modding SA, i would suggest using a Downgrade patch. To downgrade to V1.00, V2.00 crashes your game when modded, VC and III dosen't have a "Version". Only one, which works on modding/editing always.
I hope this helps. Good luck

Small tip: Start small before making a TC. Like a simple t-shirt change. Or a ped change, something like that. Then work your way up
This post has been edited by ManDog on Thursday, May 24 2012, 12:46