You have your water, your pots or jars so now you need something to dye. For practice you can try anything, but better to start with scraps of protein fibers (wool or silk) or cellulose fibers (cotton rayon or bamboo). This is about re-purposing, so think about using some of your old cotton T shirts, pillowcases or sheets. If you have silk undies you don't mind trying that's great. Me, I'm not fortunate enough to wear silk undies! But if you DO have them, maybe you want to see if you can change them. Also, if you have wool fabric or roving or just clipped wool then you have stuff to dye with.
Some fabrics will have to be prepared for dyeing(PFD). All fabrics should be washed and rinsed well. Maybe put through a second rinse. If you are going to dye these fabrics right away, no need to put in a dryer because wet fabric should be placed in the dyebath, not dry. It will start to absorb color right away and usually will not give you that blotchy effect. If that blotchy effect is what you want, however, use dry fabric. Silk and wool generally don't need anything else done to them before dyeing. If you plan to try eco printing, there are a couple of things you need to do before putting your fabric in the dye. We will get to that next.