Mick , You need to match your line to your rod , if as you say you have a 6/7 weight rod it will cast a 6 or a 7 weight line . A 6 weight can be cast just as far as a 7 weight in fact I believe the world record casting distances are done with 5 weights.The number (6,7or 8etc) is the weight of the casting section of the line , a rod needs to be loaded (pulled into a bend) by the weight of the line to be able to launch it forward , a 4 weight line is too light to "load" a 9weight rod , they are stiffer and need a heavy line to load them.
A 6 or 7 will be enough to load your rod without overstressing it.
When you are fishing though a 7 weight is probably the optimum weight to use assuming you are fishing for rainbows in reservoirs or stocked lakes.
On a perfectly still day a 5 or 6 weight would be ideal choices, they are light , cast just as far as heavier weight lines and land gently on the water so as not to splash and spook the fish , in a gale they would blow all over the place and be extremely difficult to cast , a 9 weight is far better in these conditions but is heavy to use all day and lands with a thump on the water, but you can't cast a 9 weight on a 5 weight rod and visa versa. so to save buying 6 rods and 6 lines most folk get the happy medium of a 7weight , heavy enough to cast in a decent wind yet light enough to lay gently on the water and control easily.
As for which line to get virtually every fly angler I know (and there are a lot of them) have a floating and an intermediate as their first two choices of line.
Most lads I know take two rods , one set up with a floater , the other set up with an intermediate , then carry a spare spool for their intermediate reel with a sinking line on it.
There are lots of reasons why an intermediate line will sometimes be better than a floating line , if there is a strong breeze blowing it can move a floating line sideways at quite a speed meaning no matter how slow your retrieve the fly is always moving fast and rising to the surface , an intermediate allows the line to sit just under the surface and be pulled at the speed you want without the wind affecting it , intermediates can be better in flat calm conditions too , causing less wake when retrieving .
Sinking lines can be deadly too , if the fish are on or near the bottom a sinking line with a very short leader and a floating fly is so effective it has been banned on some waters , cast out let the lot sink and wait .....that's it!
When you start to get into fly fishing you can make it as complicated or as simple as you like , but to get the best out of all seasons weathers and conditions you need to learn which lines to use when and how.
For the next few months (assuming the temps rise to normal for this time of year) a floating line should be your first choice , if it's blowing a bit and you are struggling to control it , switch to an intermediate .
Ray.
