Once you've drawn a plan and have permission, you can start looking for wood for your clubhouse.
A clubhouse can be built with just a few different kinds of boards, all held together with a few sizes of nails. It's a good idea to learn the carpenter words for the boards as they are used. This is what the frame of a possible clubhouse might look like, and what the pieces are called:
The floor is held up by two 4" by 4" thick beams called foundation sills that are 8 feet long. These support seven 2" by 4" thick floor joists 6 feet long, which then support the floor boards. The floor supports the walls which support the roof. To make them strong, the walls are built with studs, plates and other pieces; all from 2" by 4" thick boards. Thinner boards or plywood then cover up the 2 by 4s. I'll show you how to put all this together in several steps!
Most nails come in one-pound or five-pound boxes. For starters, get a couple of 1-pound boxes of 6d, 8d and 16d construction nails called "coated sinkers". Exterior galvanized nails are good for nailing on trim boards or siding that will get rained on. These are the kind used in house building everywhere. There are a few other kinds of nails or screws you might be using as well:
Next: How to find wood and other stuff ...