You will need to purchase some 2X4's. Make your 3 frames from top to bottom with studs with a top board and a bottom board to nail the studs onto. The studs can be 24 inches apart. The fourth wall will require an opening for a door. Use the corrugated plastic for the walls and the roof. It comes in various sizes and lengths so think about looking at those sizes before you start to build so that you have no waste. For the ground level for the walls to sit on you can put in 6" inch timbers and screw the bottom of the walls to that so it is strong in windy situations. There are special screws you use for the corrugated plastic so be sure to get those too.
Here are some sites with various plans!
Here is one way: https://www.hometalk.com/30431460/cedar-4x4-greenhouse