Panorama question

Whenever I try to stitch a couple of shots together on Photoshop I always find that either the Cylindrical Mapping won't work or my shots are so bent out of shape that I don't have much of a panorama after all.
Is this because I'm not taking enough shots in my panorama and I don't have enough overlap.
Or is this becuase I have the focal length wrong 18mm.
I know there is lots of $$$ I can spend to get this perfect by buying software and special tripod fixture but I just want to keep it simple and do the odd panorama handheld with my kit kens.
Cheers
Is this because I'm not taking enough shots in my panorama and I don't have enough overlap.
Or is this becuase I have the focal length wrong 18mm.
I know there is lots of $$$ I can spend to get this perfect by buying software and special tripod fixture but I just want to keep it simple and do the odd panorama handheld with my kit kens.
Cheers