OK, for all the folks reading this and who are relatively new to fishing, please do not use fluorocarbon as a mainline on your reels. Regardless of what you may have read, seen or heard of, fluorocarbon is one of those marketing jokes gone awry. You can and may however use a short length of the stuff ..say 12" to 48" as a leader between your mainline (monofilament or braid) and your live or artificial bait. The small calibers of fluorocarbon such as 4lbs or so are mainly used by people who think that a fish has the intelligence to become alarmed at having a piece of dental floss attached to their dinner of plastic with treble hooks. The large calibers such as 100lbs or so are mainly used by muskie fishermen who are concerned about the damage that a steel leader can cause to a fish rolling on a leader during combat.
There are literally hundreds of brands of monofilament lines and braided lines. For monos, go with a standard brand such as Stren or Trilene. For the braids again, go with the standards although most are almost the same. Don't be fooled by all the colourful boxes on the shelves...some manufacturers will go to great lengths to make you buy their inferior product. If you see packaging with beautiful holographic images that looks like it's worth more than the line, it probably is. Use common sense.