I'll let the trout guys tell you better but I can say that the answer depends on what you want to do.
There are three parts to a leader in most simple terms:
1) butt (60%)
2) taper (20%)
3) tippet (20%)
1) the butt should transfer energy from the line properly, so it should not be too thin nor too fat/stiff for the fly line. It should dissipate some of the energy in the line for the taper...
2) the taper steps the butt down to the tippet, which acts like a gentle brake before the final delivery.
3) the tippet is the final presentation of the fly to the fish and needs to match the fly (hook size) and the fish's power (or all is for naught)
A high density sinking line only requires a short leader of 4-5ft while a floating line might work best for dry flies with at a rod's length or longer for more delicate presentations.
As a general rule, use a leader that is more or less the rod length and make sure the butt is not too stiff or too fine for the end of your flyline.
The tippet should be robust enough to turn over the fly (hook) you are using, and strong enough to land the fish you are apt to hook... but no more than that to maximize stealth for more hookups.
The leader comes with a tippet extruded from the same mono filament but you will find the need to replace it often if you are fishing a lot.
For weighted flies, you might tie the fly onto a thicker part of the leader without the fine end tippet. Then when the evening hatch begins, put a length of tippet back on with a blood knot (a must) or a surgeons' knot (easier but not as nice).
For a 5wt, the range of tippet sizes really depends on the fly you are going to throw. If you are fishing bead head wolly buggers don't worry too much about a fine tippet. If you are fishing midges, well then you should have some spider web on a spool.
As far as specifics, that's where I will let the trout guru's answer. I am a saltwater and steelhead/salmon Spey dude so most of my tippet is maxima ug and 8# test is ultralight for me.