How to straf ?

Strafing is Easy:
Hold A and look left smoothly
Hold D and look right smoothly

You're now strafing.
If you meant "how do you Bhop", then just alternate between the two above, and press Space right as you're about to hit the ground and you should be able to Bhop. Or you could find a server with Autobhop and not have to worry about the jumping bit.