Decide on a color and get the highest thread count possible for the price but in reality the maximum thread count is around 600. Claims of higher counts are due to multi-ply thread where two or more threads are twisted together and incorporated in the weave. Keep in mind that in order to get a larger number of threads in one square inch, the threads get finer and finer. Finer threads are less sturdy and will fail. Another thing to consider is how well the sheets will stay on the mattress. This can't be known until the sheets are tested. Most of the time the elastic sheets won't work. Something to consider is trying drawstring sheets. They stay tight and flat and never pop off. Nice fabric is one thing but when your sheets are wrinkled and bunched up. It doesn't matter what the thread count is.
To buy bed sheets, determine the size of your bed and the sheet sizes you will need for it. Then, figure out the colors or patterns you want to have in your bed if you care about how it will appear in your room. Lastly, look at the material you want your bed sheets to have. Consider all these when you make your purchase.