Solid advise!!
You said that you are going to use metal for the roof. There are a couple type of metal out there, but they all install about the same. With metal, you do not need as many rafters. Spacing of the rafters is dictated by the purlins. My rule of thumb is the space my rafters 4 feet apart on center, if using 2x4's on the flat for purlins, which gives you a lot of wood to screw into. If you space your rafters 6 feet apart, you use 2x6 purlins on the edge. 8 foot spacing means 2x8's on the edge. Purlins should be every 4 feet or less.
The other thing to factor in is how far out will your roof extend? For me, I use 2x6's for 12 feet of span, 2x8's for less then 16 feet and 2x10s for more then 16 feet. After that, I'm going to use trusses or add a beam with a post. This is not for snow loads, and it's just how I like to build things.
For posts, a 4x4 is plenty strong enough to carry the load, but with modern pressure treatment, they are unreliable and very likely to bow or twist on you fairly quickly. 4x6's will give you a better chance of remaining straight, but I recently saw a few that had bowed significantly in just a year at a clients house. I use 6x6's for all my jobs because I never have to come back and fix anything. They are heavy, but well worth the effort once you have them in the ground.
Flashing is also really important and something that takes a little effort at the beginning to avoid long term headaches down the road. Ideally, you want your flashing to be under your siding of the existing building so water runs down the building wall, over the flashing and then over the metal roof and then out over the building. This sounds simple, but it's often the source of a lot of issues.
Since you are new to this, you might want to get some graph paper and draw out your framing. Then take a picture of it and post it on here for everyone to review. This is very basic framing, but it's also where a lot of the DIYers skip a step to get it done quickly that they really shouldn't have skipped.
