There are a number of good websites out there where you can order ingredients. I used to do this exclusively until I moved somewhere with a homebrew shop, and I still do when I need specialty ingredients. The brew shop by me is quite small and has a limited selection.
Here are a couple of good online shops.
Austin Homebrew
Midwest Supplies
I'd say go with a packaged kit to start with. All of the ingredients are measured out for you and the recipe was made by someone who knows what he's doing. Make something standard and simple, and follow their instructions.
Once you get a feel for the technique, then you can get creative. You can get individual ingredients and make your own concoctions.
I tend to look in a recipe book for some inspiration, then tweak it to my tastes. For example - I really don't like hoppy beer, so I will tend to substitute lower alpha acid hops than what's called for.
Enjoy!
PS - Is it really called Mr. Beer? That's classic. If I was ever a superhero, I'd want that to be my name.