Circular needles: Knitting of garments like socks is often done using circular needles. You can move on to these projects with some guidance after having mastered the use of normal knitting needles and the basic knit and purl stitches.
The patterns and designs: Once the basics of knitting and purling are mastered, you will find that there are many patterns and designs that you can use when you are on a knitting project. But do ensure that you are confident of the knitting process and the process needed to pick and repair a dropped stitch before you start on these patterns. This way the end result will be really worth it.
Often you will find that you get wool that already has some kind of patterning to it and this type of wool will make the whole garment that you are creating out of it look pretty without that much effort. You can start with these if you are still not confident that you have mastered the art of knitting before going for finer and costlier wools.