Well, if you want my honest opinion, I say don't become one. You don't need to be a "freegan" to conserve energy, and you don't have to cut out all junk foods and such. If there is such thing as a "half freegan" label yourself as that.
If you happen to cut out junk foods permenatly, your body will probably go through a major "crisis" because that's what the body is used to eating. I'm not saying your gonna crack after 2 weeks and eat a bag of chips, but it'll probably be terribly hard, like quitting smoking is for older people, (and young ones too) Your body craves it, so if you do cut it out completely, you might be a bit stressed.
It's great you have good intentions, but having junk food once in a while isn't bad at all.
Your conserving energy thing is great too, but still ride in a car once in a while, okay? xD
Oh yes, and you also said "freegans" don't buy things they don't need, I do and don't agree with that. If you have money, it's okay to treat yourself to a new CD or lipgloss or something, but if the only things in life we "need" are air, food, water and shelter, it's gonna suck a bit, right?
Good luck with your choice, and your intentions are great. :]