It's going to really depend on what you want to do. Do you want to cast just square blanks to be turned, or do you want resin saver molds.
Cheap and easy I'd say start with casting craft resin from Michael's. Sign up for their email club and they will email you a coupon each week. This week is 40% off on one item. That will reduce the cost of the resin to a more managable level. While your there, you probably also want to pick up some pearlex powders. There are three sets of different colors. You'll also want some sort of colorant, some people use testors enamels, others use transtint dyes or other mixes specifically formulated for resin.
Then you will need a mold, either a cutting board mold or a silicone mold made by gadget or NewLondon88. Cuttingboard mold is obviously cheaper to make, but you do have to make it yourself.
After that, you will need your disposables. Mixing sticks, resin mixing containers, rubber gloves, etc....
From there it's up to you how you procede. You can warm your resin, or warm your mold, or both, you can vacuum the resin down in a vacuum chamber to degass the resin, or you can cast under pressure.
Read the thread from ItsVirgil regarding his no pressure casting method. It takes a bit more work, but saves you the compressor, the pressure pot, and other incidentals.
