Buses for Hua Hin leave from the Southern Bus Terminal (Sai Dai Mai) in Thonburi (left bank of the river.) I wouldn’t bother booking ahead, since they leave every half hour or so, and your travel period doesn’t fall on a long weekend. (While August 12 is a holiday, that’s too far away to have an effect.) You should be able to turn up and get the next bus, or at worst wait half an hour. I would stick to 1st class buses (the majority) and avoid the 2nd class ones, because these typically stop to pick up passengers along the way and can be frustrating.
If you’re staying on Khao San Road, it’s easy to get a city bus to the southern terminal – check this link for buses that run between the Democracy Monument (the big road just a block from Khao San) and the Southern Bus Terminal, e.g. number 511. A taxi from central Bangkok shouldn’t be more than 200 baht, and probably much less.
Alternative ways to get to Hua Hin are the train, although that’s generally slower, and a minivan from Victory Monument. It’s easy to get to VM since it’s on the Skytrain line, but a bit more complicated to find where the minivan you want leaves from. (See the map on this page.) Also, there may not be much room for luggage.
All in all, I would say the bus is the best option.