With all boards cut to the proper length, the next step is to joint one edge of each of the boards. This can be easily done with a jointer.
If you don't have a jointer, there is another option. Try using a table saw jointer jig. This simple jig allows you to create a smooth, flat edge that is perpendicular to the flat sides of the board using only your table saw.
Jointing one edge of the boards is vital to having a clean, seamless tongue and groove joint.