When choosing fishing line, experienced anglers look for two things; the breaking strain (the amount of weight the line can withstand before breaking) and the diameter. These are generally speaking the two main factors that differentiate expensive lines from cheaper ones, i.e. the cheaper the line, the thicker its diameter will be.
There have been many advances in fishing line over the past few years, resulting in specialist pre-stretched lines to reduce diameter, and lines made of new materials such as fluorocarbon which is near invisible in water, but for most general fishing situations an inexpensive nylon monofilament (mono) line will be fine.
That said, you only really need to be concerned with the breaking strain when buying line, and as a general rule if you are just starting out, I would suggest that you use line of about 3lbs+ for float fishing situations where you expect to encounter only small fish, and up to about 8lb if larger fish are possible. If you can, though, ask other anglers who fish your chosen venue for advice on this, as well as the baits and hooks that work best.
Line for loading on to reels is sold in spools of 100 metres. Depending on the breaking strain you can get 100m of perfectly good line for less than £5 per spool and it can be significantly cheaper in the long run if you buy bulk spools of a few hundred metres.