I know that her postal code goes on the seven small boxes on the upper right side, and that the stamp/stamps go on the upper left side, but not much else. I can write her address in Japanese, but is it preferable to write in English instead? There are five lines
in the center of the envelope below the postal code boxes and the place you put the stamp, so should I write it there? How do I write it? I don't know where to write my own address, either. I'm sorry that I'm asking so many questions, but I want the letter to get to her. I also don't want to prove my stupidity. xD; So, um, help please?

Name of Perfecture Name of Region Name of Block
Address

Name of Receiver + Kanji: -Sama

So here, we see that in Japan, an address usually started with the post code, then going from the wider area to the more specific ones. In my country, it's backwards. We started with the name of the receiver and end with the postcode. On Japanese style envelope, you have to write it top down. From a bit at the right side from the center of the envelope start writing the post code from top to bottom, then work your way to the left by specifying the address and then the name of the receiver.

A fact that you should know is that Japan's streets barely have names. So, sending a letter with a namestreet is not a viable way to do. Instead, you have to use blocks. For instance: 5-choume 5-ban 5-go (or usually written 5-5-5 for a shorthand). It means the 5th part of a region, the 5th block and the house numbered 5.

The next thing you should know is the addition of the kanji 'sama' after the name of the receiver. It is crucial to do this when sending to a Japanese, because this indicates honorifics for the receiver.

Next, to write the sender name and address, you have to write it on the back of the envelope. For Japanese style envelope, start on the right top side. Add your name and address on that side. Starting from the right most column for the Postcode and working yourself to the left. Beware when writing the name of the sender no honorifics is used. Just write your name plain. Writing no honorifics on your name made the impression that you lowers yourself to the receiver thus it is the polite way. (actually this rule also can be apply when conversing. You have to add some honorifics to the other party like: -san, -sama, -kun, -sensei, -chan, etc. But you should never try to add honorifics to yourself. It will sound pretty weird.)

The example shown is an address of a company, but when you are writing to an individual, you write the receiver's name in the center and put 様 (sama) at the end of the name. You are to write your own name and address on the other side of the envelope.

But it's alright to write everything in English instead of Japanese. Just remember that the order of the address is different depending on what language you write it in.