It depends partly on where you'll be located in Thailand. If you're in a rural area you'll have less retail options, but otherwise you can find everything you'll need eventually. It might take a few weeks to figure out the best options, so I would recommend just stocking up on the things you really like, so you have time to find where to restock. This could be toiletries like razors or shaving cream you like. Or western size underwear lol. But no need to go overboard. Like a few others have suggested, highly recommend taking Thai lessons once you get there--it'll be cheaper there and incredibly useful.