Re-Beginner,
Normally you attach the leader to the fly-line, best way is with a loop-to-loop connection. You can tie a perfection loop in the thick end of the leader and most fly-lines have a welded loop. If your fly-line does not come with a welded loop, then you can attach a short leader butt to the fly-line; use some Amnesia shooting line or heavy mono (15# or 20#), attach it to the fly-line with a nail knot, and then about 3-4 inches out tie a perfection loop - this is the fly-line loop you will connect to the leader loop.
If you are going to tie the tippet directly to the leader, a double or triple surgeons knot is one of the best. A lot of folks will tie another set of perfection loops, one in the thin end of the leader and one on the end of the tippet, and again do a loop-to-loop connection between the leader and tippet. Some furled leaders come with a loop on the butt end to connect to the fly-line, and a really small ring on the other end, so you can tie the tippet dierctly to the ring using any knot you like - e.g. you can use the same knot you like to use when you tie the tippet to your fly. Using the loop-to-loop or a furled leader with a tippet ring wilol eliminate cutting off a small part of your leader every time you need to tie on a new length of tippet - makes the leaders last a long time. Furled leaders can cost 1.5 - 2 times as much as a tapered knotless leader, but they last a really long time and they are exceptionally strong. I usually get at least a full season out of one furled leader, fishing up to 60 days a year.
I would recommend a 6ft-7.5ft leader and the leaders should be a 5X-4lb or a 6X-3lb; for very clear/slow water where the fish are pretty easily spooked, sometimes you have to go to a 9ft leader in a 7X-2lb. The tippet should be no heavier than the leader - again, 4lb for faster water with some color, 3lb for normal, and 2lb for slow/clear water. Mono is the cheapest tippet, but more expensive flurocarbon can be better for nymphing since it sinks faster and the fish have more trouble seeing it under the water. Remember, do not use flurocarbon for dry flies - fluoro sinks! I usually use 3ft-4ft of tippet, depends on the depth of the water you are fishing, sometimes I use up to 6ft of tippet. Basic rule of thumb, fast water - use 50% more tippet than the depth of the water, slow water use the same length as the depth of the water.
Here are a couple of links with more information:
http://www.flymartonline.com/article215.html
http://www.animatedknots.com/indexfishing.php?LogoImage=LogoGrog.jpg&Website=www.animatedknots.com
Hope this information helps.