I know this sounds stupid, but I’m a little confused. I’m teaching myself to play piano and I’m writing a music sheet. It’s a little hard to explain what I’m asking but uhh…for example:
There’s 5 A keys on my piano. How do you let people know which one you want them to start with? What if you start with the first A key and they mistake the one you place on your piano sheet for the 3rd A? I’m confused 
You are worried about running, when you have not yet learned to walk – or crawl. You, as the composer, must learn to read music. Then the performer reads what you have written, and can play it. Yes, there are many occurrences of each letter – just as a hotel has many rooms that are the same shape, but on different floors. Or – each street has the same house numbers – but they occur on a different street.
I advise you to go the www.practicespot.com and learn to read music. This is critical if you are going to play an instrument yourself, and if you wish to communicate your compositions to other musicians. You will soon see how our notation system helps us know WHICH pitch we want, in which octave.
Second step – which might be days or even weeks later – go to www.makemusic.com and for ten dollars, download Finale NOTEPAD. You can enter you compositions on this program, save them, and play them back. When you are satisfied, you can print the sheet music for OTHERS to play. You can also go the Finale website and download many compositions and help files that will help you advance you skill.
Since 1971, I have taught THOUSANDS of students to do this. It is exciting, and NOT difficult – much easier than math, or learning another language! Please do NOT call yourself stupid – you are merely frustrated, and need a teacher to show you the way. I have been a teacher of music for many years – I hope this has helped you. Write back in the future with other questions if you are stuck – I will be here . . .