Okay, I have medium length hair it's thinWhat's the best type to get at walmart for $30 and under, and it heats up really really fast and works really good and it's the thin type(:
Make a paste of baking soda and warm water, and apply as if a polish, then simply wash it off.
A $40 flat iron won't be any better than a $30, and I personally think there are several really quality flat irons for that priceI would recommend the Ceramic Tools CT2555 1-inch ceramic flat iron (I've linked to it below)Make sure that you do not set the heat to the highest settingBecause you have thin hair, you could cause a lot of damage that wayDo buy a heat protecting spray for your hair and use it before you start using the flat ironIt will be called heat protecting spray, or thermal spray, or something like thatIt is EXTREMELY importantOtherwise your hair will end up of feeling like straw, trust me on that oneWhen straightening your hair, split it into sections and iron each section, making sure not to keep going over each section several timesThis can also cause damageAlso make sure your hair is completely dry before straightening your hairIf this is an issue, there are wet-to-dry irons availableHave fun!