The two most popular and largest townships are Bang La On and Bang Niang, both of which are suitable for backpackers and offer a good selection of reasonable accommodation at reasonable prices.
Have a look at this comprehensive list here:
which will give you a good idea of what's available, where and at what price.
The Khao Lak Youth Club (not really a Youth Club, just a name) has a good reputation and appeal for young people, including backpackers.
Any more info after you've looked at these, just ask away.
BTW hope you've got your visa sorted. If not see thaiconsul-uk.com/downloads-and-visas.aspx
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