Just as with any other kind of knitting, there are numerous ways to cast on stitches for toe-up sock knitting. The best method for you is going to be found after trying a variety of options, but I like one that's known as the Turkish cast on, mostly because it is relatively easy and doesn't require the use of waste yarn as some provisional methods of casting on for toe-up socks do.
You'll probably want to practice a couple of times before you use it on a real sock, because you're working into loops so the tension can come out pretty uneven. But it's quick and easy to cast on this way, so you won't mind too much if you have to rip it out and try again.
This method can be done with two circular needles as shown, or start with two double-pointed needles and distribute the stitches onto one or two more as you go, depending on your preference for how many needles to work with.
To begin, align your needles with the points facing to the right and put a slip knot on the lower needle.