It's starting to annoy me, I want to make a fort but i cant get them to fix together no matter what

When you place the first piece, set the origin and angles to 0, So set x,y,z to 0,0,0 and all the angles to 0,0,0. Then set the snap to grid to 16. That's fine enough to accommodate the majority of pieces, but large enough so that it's not too tedious. Unfortunately there are some pieces that won't cooperate. There aren't many, but if you find one, set the snap to 1. That way the position will still be an even number and other pieces will connect to it.