The main aspects to look at when choosing bathroom window curtains are color, style and fabric. After that all you have to do is measure your windows accurately, buy the right size and you cannot go wrong.
If you have a plain white bathroom (as most of us do these days), it is important to add some color to your room so that you can avoid it looking bland and window curtains are a great way to do that. The more neutral your bathroom (including wall tiles and floor) the more choice you have when it comes to color but if you do have to match up any fixed elements in your bathroom, take samples or spare tiles with you when you go shopping so that you can get the color of your curtains just right. If you can't do that, use a paint color chart to find the nearest color and use that. It will save endless trips back to the store with curtains which don't work.
As the bathroom is such an informal room, you need window curtains which are as simple as possible. Avoid complicated headings, ruffles and flounces and choose a simple gathered or tab top heading. Bathroom curtains should be short rather than full length as this adds to the informality (and in any case you will probably find you have no room for long curtains).
A plain or patterned cotton fabric works well for bathroom curtains or you could use a polyester cotton mix. You will need to wash your curtains frequently, so choose good quality thick fabric which will not fade or go limp after a few times in the washing machine. There are waterproof curtains available which are specially designed for bathrooms. They are easier to care for than cotton curtains as you just wipe them clean but you will find your choice in designs quite restricted compared to the range of cotton fabrics.