Seek medical attention for yourself before attempting to do anything else. If you’re not badly hurt, documenting the accident scene through photographs can prove extremely valuable to the investigation. Eyewitness accounts (or at least contact information) from accident onlookers can also serve as powerful evidence.
What you should not do is admit fault to anybody at the scene. Limit what you say to statements of facts to the responding law enforcement officer. Avoid speaking altogether to insurance adjusters and attorneys representing the trucking company and other potential defendants. Consider securing legal representation of your own, or at least speaking to a lawyer about your case.