Work in a well ventilated area and protect your hands/skin and eyes from caustic fumes and materials. If you're going to grind or burn the plastic wear a respirator. I can't stress how important safety is.

Since I have no idea what kind of plastic you are wanting to use nor your desired result I can't advise much.
Tap Plastics are a good source for materials, glues/solvents, and other items. They are also very knowledgeable about plastics.
Play with your materials before you start on the final project to know your medium well. You'll need to know everything you possibly can about how to get your plastic to conform to the use and image you want.

Hmmm...Maybe I should explain a little what I'm trying to do. I had an idea for making these abstract art pieces. They'd be sort of large, one's long and skinny and loopy, sort of like a garden hose, the other is a flat round thing about the size and shape of a toilet seat. I have specific designs in mind for both pieces, I'd like to get them to the shape and size, color I want. I'm just not sure how to go about starting, other than sketching out the specifications. I'm willing to do a fair amount of research to learn. Maybe that'll help.