the best way to get into modding and the way i did it was learn to make car and bike skins, download txdworshop from here and spooky's img tool then use the imgtool to open the gta3.img in the models folder then search for infernus.txd and pcj600.txd then extract them, open them using txdworkshop and extract the images you want to edit, body, interior, engine etc
use photoshop or the gimp or even paint (i started in paint) to edit them, save them to png and import them back into the txd then use the imgtool to import your new txd and delete the old one from the img, make sure its named correctly, if your deleting the pcj600.txd and putting in a new one then it can't be pcj.txd i must be pcj600.txd.
then go ingame and find one and you'll see your new skin. (a tip, go into carcos.dat and replace the vehicle your editing with "vehicle-name 1,1" so it shows up as white and you see exactly what you've made.
theres high demand for skinners at www.gtastunting.net/forums though that place is full of ass holes.
final tip, make sure you back up what ever you edit incase you do something wrong.
and if you do like that and get good at it, you'll prolly find out about other programs over time but, 3ds max and Zmodeler are the most commenly used ones to make gta models oh and google sketchup sometimes.
wait!
uhh what kinda images did you mean? like i downloaded this neon mod it replaces the shadows of cars and bikes...
but this stuf i wanna add more colours like flurasent etc. i made one like a flurasent but when i try ta replce it withe the current shadow it says: cannot read ..... i donno what comes after read i fogat
i tryed it a month ago
and i download this zmodler but its not working! help!