After-market seats come in many different styles and options. The two basic ones you will need to familiarize yourself with are bottom-mount and side-mount. This means that the mounting bolt holes on each side of the seat are either on the sides:
Or on the bottom of the seat:
You will then need to consider brackets and sliders. brackets are the mounting point to the floor of you car. Sliders sit on top of the brackets and bolt to each seat to allow them to move back and forth. Brackets are usually car specific, whereas, most sliders are universal. Here is a picture of a bottom-mount set up with sliders:
And here is one of a side-mount configuration:
Additionally, you should look at the type of adjustment the seat offers, as well as, the ability to add harnesses or to adapt your seat belts. Material (leathers vs. fabrics), color schemes, and price play a big role in choosing a set up too.
Hope this has helped and good luck to you!