I have just picked up a couple of Shimano reels and want to put some decent mainline on them. I have not really got a clue about the type of line I should be using.
This does depend on a number of different factors.
If you are fishing at short to medium range in clear water then a flurocarbon line would be a good choice. It is much heavier than mono or braid and so will sink like a brick. It is also virtually invisible which is a very good trait when fishing is clear water.
If you are fishing at long range, you could go for a mono or fluro coated line. Both are lighter and will cast much further. Again if the lake is clear you should probably lean toward the flurocarbon coated to make the line as unobtrusive as possible.
Many lakes have weeds and snags, in this scenario you want something that is abrasive resistant. I would increase the breaking strain for longer range to ensure the line does not break when you get a take but is also light enough to cast the longer distance. If you are fising at shorter range you could opt for a braided mainline and a flurocarbon leader which will provide you with a very strong mailine and an invisible leader to best display your rig.
I use Gardner Insight GR60 12 lbs as my go to mainline which I have always been very impressed with.