I have a Kent brand fixie bike with 700x38c tires and I am looking for a new front wheel what size should I buy
A tire that fits an internal rim width of between 17mm and 23mm wide.
Searching for bicycle tire width rim guide will get you more information. So anything from 700 by 17mm internal width, to 700 by 23mm internal width will suit you.
If you think you likely use skinner tires in the future, stick to the smaller end of the range. If you think you'll go wide, stick wider.
Any 700C or non-plus 29er wheel will be able to physically mount it, but there's then the consideration of rim width relative to tire width. Rims for road bikes are usually 20-21mm wide or so measured across the outside. You ideally want something more like a hybrid/touring or XC wheel for a tire that size, with an outside width of 22-25mm.