Turn on the controller and truck power. Turn your fine tuning steering adjustment to center. Adjust the turnbuckles to set wheel alignment. You can eye ball them or use a ruler and measuring tape to line them up.
I'm sure there is a tool to do wheel alignments but I haven't found one.
Are you front wheels parallel? You may have to adjust the turn buckle to make them parallel. Also are your back tires turned in a little and the same amount? If your back wheels are toe in a little (I think 4 degrees) it will help it to stay straight.
From your steering servo follow the linkage and tie rod to the wheel hubs. The tie rod should be threaded with a spot on it to put a wrench. Turn the tie rod one way or the other until your wheels are straight.