Choosing a pickup is important. Why?
-They're not cheap. Expect to spend $70-130(USD) on a good pickup.
-It'll change the way your guitar sounds. Look for video reviews with good quality audio to help determine what you need. Seymour Duncan also provides audio samples for their pickups. http://www.seymourduncan.com/support/audio-samples/tele_jaguar_and/
Make sure you like the sound before you buy it. Don't go off of text reviews alone(although you should read those too). Everyone's opinion is different.
I chose the Little '59 because I wanted a humbucker(vague, I know...), and there were good reviews on it. Also, I was pleased with the way it sounds.
I recommend you buy your pickup new, and from a place that you trust. I am getting more and more unsatisfied with Musician's Friend(everything I order is backordered...) so after being informed that my pickup was going to be in stock three weeks from when I wanted it, I canceled the order, bought the same pickup at Guitar Center(online) and it came in by the end of the week.
Also, make sure you know where the pickup is supposed to go(not just what type of guitar it is supposed to go in). Don't screw up and buy a neck pickup and try to put it in the bridge.
That is, of course, if your guitar doesn't have interchangeable pickups. Like a Tele.