I need to build a gazebo 12/12 octagon

What is the question specifically? I would suggest a post and beam type structure where you set 8 posts in the ground and hang the rest of the structure on them. Attach a rafter to the top of each post and have them meet in the center in a point. If the rafters are too far apart you can nail purlins across them to support the roof deck.

Hello, You should look into the pre-made type of gazebo. You can find wood and vinyl kits. Much better than trying to make by scratch.