well somewhere ( im not sure were i'm new ) there is a forum which explains all to modding but her's a brief
to install a skin
1-go to your root directory (where gta-sa.exe is)
2-open models->player.img
3-open player.img with an img tool http://www.thegtaplace.com/downloads/file.php?id=552
4-read the readme and it should say which file to replace
5-using img tool find the .txd and make a backup of it then delete it from the archive (player.img)
6-Using winrar you extract the file given in the rar archive
7-add the file you extracted to the player.img and then do rebuild archive in one of the tabs
8-start up San andreas and have fun
to install a car its the same principle but instead of player.img its "gta3.img"
Some cars may have a readme showing a handling line, carcol line, vehicle line or carmod line
these files are found in the data folder in the root directory of san andreas
the are (respectively) handling.cfg, carcols.dat, vehicles.ide, and carmods.dat
well thats as brief and detailed that I can be
if you have questions message me