Don't assume that a saw will make accurate cuts right out of the box.
Crosscuts: Set the miter gauge at 90 degrees, mark an X on a board, and crosscut through the X. Now flip the right-hand piece so that its marked side faces the opposite way. Butt the cut ends together, and set both pieces on edge on the saw table. If there's a gap at the joint, adjust the gauge and repeat the test until the gap disappears.
Rip cuts: Slide the edge of a straight ¾-inch-thick board into the table's miter slot. Place a block of wood next to the board, and slide the fence against the block and lock it down. If the block doesn't slide freely between, and remain in contact with, both the fence and the board over the fence's entire length, adjust the fence and try again.