Reddit reviews The Art of French Horn Playing (The Art of Series)

Don't close up your throat, and don't force your horn into your face when you play, It will exhaust the muscles that form the embouchure and weaken them by restricting blood flow. Raise the tongue when you blow. It will take practice and endurance. Warm up with fundamentals. Id recommend The Art of French Horn Playing for any horn player

Get a private teacher and/or start reading up. I can recommend the following:

• Art of Horn Playing - Farkas
  
• Carved in Stone - DeRosa
  
• Horn Handbook - Reynolds
  
  The DeRosa/Reynolds are more advanced, while the Farkas talks about basics -> advanced. Highly recommend the warmup/exercises in the Reynolds and DeRosa though, the farkas can be....strenuous. Your high school band director should know enough to help you with the basics as well.
  
  Hit up your local college's music library; they may have copies of the above, and if nothing else you can sit there for half an hour and skim or copy down the exercises.
  
  Credentials: Licensed Music Educator, 2nd Year in Master's in Performance
  
  Edit: Generic rule: If you feel tight, you are probably doing something wrong. Relax!