Welcome! I would suggest starting with some inexpensive, plain smooth yarn. Basic acrylic yarns can be purchased at most craft stores (JoAnn's, Michael's, Hobby Lobby, etc), or if you would prefer wool visit a local yarn store (frequently abbreviated LYS on the forum). In either case you want something smooth and medium weight (look for worsted wight) so that you can see the stitches easily. Avoid the fun novelty yarns (such as eyelash) and highly textures yarns (such as Lion Brand Homespun) at first because they don't let you easily see what you are doing. "Ribbon" and 100% cotton yarns are also not very good for beginners because they do not have very much elasticity, making them more difficult to work with. Good luck!