Download TXD Workshop (use google for that.. ), then open it , click on "Open IMG" then navigate to gta3.img ( normally in GTA San Andreas folder/models ) open it. Then for example lets say you want to change your NRG-500 look, As you see in TXD Workshop on  the left side are files and files and more files .. so scroll down until you see nrg500.txd , click on it and you see 4 files open: brakes, body , metal and decals files. Second file is the body template , so what you do now is click on the body file , click on Export , click on PNG , navigate to whereever you want it to be exported and click open. Then open your picture editing program (Paint , Photoshop , Photofiltre .. whatever it is) and start to mod it ..
Also use Google and YouTube for better and simplier tutorials.

when i am done finishing my mod how can i put it in gta sa ?

when i am done finishing my mod how can i put it in gta sa ?

Open the gta3.img in TXD Workshop as you did previously to get the body file , find nrg500.txd again , click on the file you want to be replaced ( may be not needed ) and click on Import.
Make sure you save edited file as .png forgot to say that before.

OMG ?! when i try again to open gta3.img it says
List index out of bounds (0) i tried re-installing workshop but its the same
how ca i fix it ?!?!?!

Close your game while you open it

Since this is extremely helpful and such, from p1nts explinations, can we have him make a full detail one and sticky it for future refrence?