To get started stress-free, first buy a small kit that comes with a sweater or scarf pattern already included - it'll tell you exactly which needle size to use and you won't have to struggle decoding yarn labels. If you want more flexibility, just memorize this trick: the needle should be roughly as thick as your yarn - you can test it by running the yarn over it, it should slide smoothly but not slip off either.
For the material, bamboo is honestly safer to start with because the yarn doesn't slide everywhere, unlike metal where everything escapes the second you're unsure about your tension. Bamboo is also less nerve-wracking when you're casting on stitches - with metal, a dropped stitch is a real panic. After that, once you really get the hang of it, lots of people prefer metal because it's faster, but there's no rush for you.