How can I get rid of the white crease around my armpit/white spots on my backside/white spots between my shoulder blades?

You can use a stand up tanning bed holding your arms straight out to your sides or up in the air or lie on your stomach with your arms straight above your head in a standard bed.

Self tanners are always an option to, give one of these a shot:

http://www.lotions-for-less.com/home.php?cat=271

Ive always used stand-up beds alternately w laydowns to eliminate white-lines/spots or areas less exposed to the bulbs in standard beds. They also eliminate 'bulb lines' when you are first building your tan :)

stand up beds are great and so are spray tans, but if you have white spots, you can also use a self-tanning cream and lightly spread it on the lighter spots to create a more even tone and color.