A French braid gathers the hair in bit by bit rather than starting with it all gathered together.

How to do a French braid:

1. Find someone with medium-longish hair. Brush their hair back and get a ponytail holder (hair tie, rubber band, whatever you call it) handy.
2. Take a small portion of hair from the center, near the crown of the head. Split it into three parts. Cross the right strand over the one in the center, then the left over the one in the center. (n.b. that's the one you've just moved from the right)
3. Keep the two strands on the left in your left hand. Drop the rightmost strand and with your right hand, add some hair to that strand and cross it over the center.
4. Now hold the right and center strands in your right hand and add more hair to the left strand with your left hand. Cross it over the center.
5. Keep doing that until you have all the hair from the head gathered in. Once you do, you just do a regular braid (right over center, left over center, right over center...) until you're nearly out of hair; then you can tie it up with a hairbinder or ribbon.

N.B. A good way to keep it neat is, when you are picking up more hair to add to a strand, running your thumb underneath all the hair you want to add--if you do this carefully you can get those neat straight lines.