sister the first roof joists to the existing shed. Then attach it with lags or carriage and nuts.
Then extend your roof over the area intended. Periodically use 2x4's to act as posts until your ready to attach the real 4x4 posts in front.
I would go out and buy a plumb bob. Remember the heavier the bob, the quicker it stops swinging. They are about $5 or so and then get some braided line to tie to the bob and to the point where the post will be.
Ok, measure the distance from the ground to the brace area of the porch where the 4x4 will fix to the porch.
Now cut the string this length or so and nail it to the porch roof area and then to the top of the bob. Let the bob swing and when its done, it will point to the area where the hole must be dug to sink the post. Dig the hold straight and true. When the hold is dug, line up the bob again and make sure its in the center of the hole. If this is done, then cement in the posts.
Now, I would cut the post the exact distance between the ground and the fastening point of the porch. With the 2x4 temp posts, raise the porch up about 1/4 inch heigher than you intend for it to be. So, when the posts has set and the concrete has also, you can easily move the temp post away and the porch will sit on the post as you want it.
Now, do the same with the other post and your job should be done.
PS. once the posts is in the hole and the concrete is still wet, you can use a level to find level on both sides, but remember, regardless of level, you want the post to fit as needed on the porch attachment point, so level might not be actual level. You with me???
Plum bobs are very accurate, and an old way of doing just this job. They are cheap and something to have around.
I have been told that ancient Egyptians used this tool on paramids and their buildings.