A bookshelf is the perfect first project. It is genuinely useful, it forgives small mistakes, and it teaches the core skills you will use forever: cutting square, gluing, and squaring up a box.
What you need - One 1x10 board, about eight feet long - Wood glue and four bar clamps - 1.5-inch wood screws - Sandpaper in 120 and 220 grit
The cuts For a small three-shelf unit, cut two sides at 36 inches and three shelves at the same width, around 11 inches. Measure twice. Use your circular saw against a clamped straightedge so every cut is dead straight. Mark your shelf positions on both side panels at once, so they line up.
Assembly Run a thin bead of glue along each joint, then drive two screws per shelf end through pre-drilled pilot holes. Pilot holes are not optional here; without them, the screws will split the board edge. Clamp the assembly and check it with your combination square before the glue sets. If it is out of square, nudge it now.
Finishing Sand with 120 grit to knock down splinters, then 220 for smoothness. Wipe off the dust with a damp cloth. A simple wipe-on oil finish or a coat of water-based polyurethane will protect it and make the grain pop.
That is it. You now have a real piece of furniture and, more importantly, the muscle memory for square cuts and clean glue-ups. Your next project will go twice as fast.