[Vince]

Editing skins is the easy part. You can start out with that. There are three things you will need: an IMG tool, TXD Workshop and a photo editing program. Modeling new stuff is extremely difficult if you have no prior with the program you will be using (recommend 3D studio). Square buildings won't really be a problem, but rounded things like weapons, cars and player models will be a lot harder. Especially since you'll also have to consider that you have to place special dummies (to determine where a player's hand or ass goes on a weapon/vehicle) and use special colors so the car can have different colors ingame. I think gtaforums.com can supply you with some good tutorials.