Something that seems to confuse veteran as well as novice fisherman are the number of choices there are for line. There was a time when you could only by mono line, but now you can get braided, co-polymer in a range of colors and poundage. It gets confusing! The best way to decide what line to get is to match it to the rod outfit you have already created.
The video above provides a 1-minute introduction to the basics of line selection, but it's largely based on the species you're after, which of course also needs taken into consideration.
However, Babe Winkelman at Sportman's Guides recommends basing it off your rod's action.
For light-action outfits:
This is going to be used mostly for light fish, so you can use a light poundage, such as a 4lb line. This will be able to handle a heavier fish, should you get one, but isn't overkill.
You might want to try this in mono since the monofilament lines stretch well without breaking, and they cast easy. Additionally, since light action rods are often used with bobbers, this line has good buoyancy, which you'll need.
For medium-action rods:
You can still use monofilment (for the same reasons as above) but you'll want to up the poundage to about 8lbs. This is enough to handle more than a 8lb fish, and will match the characteristics of medium-rod well. It won't be as easily broken as a 4lb line, though, so you'll be able to field bigger or more aggressive fish.
For heavy-action rods:
Babe recommends a heavily braided superline around 20lbs: this will get you good weight in heavy coverage so your line won't break, but also can handle big lake trout and other species as well. Additionally, this super braided line has a smaller diameter; so even though you're casting 20lb line it might take up what 6lb line would on your reel. And that makes a difference in the casting.
Next time I'll touch on the basics of choosing a color line based on your conditions! Hope this helps, and your outfit should nearly be complete.