Like a few said a 12Ga pump with changeable barrels is a great first or only gun. Remington 870 or Mossberg 500 are both inexpensive and very reliable.
With a 12ga you have all types of bird shot, buck shot (OO is good) as well as slugs. You can also modify it to a folding stock or pistol grip depending on your local laws.
The 12Ga pump is very versatile and VERY effective except for great distances.
Good luck getting only one gun.....
Bill