Number One: The Chain 2 Method
Start by chaining 2.
Number Two: The Chain 4 Method
Start by chaining 4.
Slip stitch into the last chain stitch (the first chain stitch you created) to form a ring. Now you're ready to create a turning chain of the appropriate height and then work your first round into the center of the chain ring.
Here you can see my hook inserted through the middle of the ring.
Here you can see my first round underway. The hole left in the center of this circle will be larger than if you used the chain 2 method, which could be either desirable or undesirable depending on the project.
Number Three: The Magic Circle Method
This is my very favorite method of starting a circle. I have a full, in-depth tutorial available here. The hole left using this method is barely noticeable at all.
Number Four: The Formed Ring Method
Did you know that you can crochet onto any ring? You can! Metal, plastic, wood -- they're all fair game. Just slip stitch right onto the ring and crochet away. If you need a crochet circle with a wide center and a lot of strength, consider using a preformed ring.