if you are OK with mess, then you can just sleep on the mattress without any bedding sheets at all. but if you do put your comfort and tidiness first, you really need a nice clean bedding sheets. there are many stuff you need to think about when you are choosing a bedding sheet for your bed. here is my advice:
1. Size. you need to choose the right size, otherwise it will be awkward to look and uncomfortable to sleep on. Sheets come in all the standard mattress sizes, including twin, double, queen, king, and California king. If you're getting sheets for a twin bed, make sure you know whether or not it's standard length or "long," as there are different sheets for both.
2. Pocket depth. you should really pay attention to the pocket depth. you'd better measure the depth of your mattress before purchasing a bedding sheet, and make sure the sheets you get are deep enough to fit over it. This is especially important if you use a mattress pad, which might make your mattress a little too tall for some bed sheets to fit over.