You don't need to have a collection of all of the sizes you can find in order to begin knitting. You'll find it handy to have a pair of 4 mm (US 6) and 5 mm (US 8) needles and perhaps a pair of 9 mm (US 13) if you want to start off with a super bulky weight yarn, while you practice. Straight knitting needles are ideal for a beginner knitter. If you have any vintage needles (metric) that your grandma or neighbor gave you, these sometimes don't have any numbers on them so it can be hard to tell what size they are.  This is where you can invest in a needle gauge - an inexpensive tool that will help you to determine the size of your needles.