I am self taught on piano. There are some notes and symbols that I am trying to learn. I have come across a music sheet that starts with Cmajor and then along the piece it changes to 7 sharps. How do you play this?
This is called a key change. The 7 sharps simply means that it's changing key from C major to C# major. Treat everything after the key change as if you're playing in C# major.
So, when the key is still in C, you'd play no sharps or flats (unless noted by an accidental), and after the key change, if you see a C in the sheet music, you'd play a C#, if you see a D, you'd play D#. Note that E# would be the same key as an F, and B# would be the same key as a C.
It has indeed got a key change , from C to C#. It could have been written in Db instead, with 5 flats!. But keeping to a 'C-ness' means that the dots stay in the same places, lines,spaces, as they were, assuming the tune stays the same. It's a trick used in a few songs to add some excitement or stop some tedium. If you are struggling with playing in that key, you could always pretend there are no sharps, thus staying in C.
Finding the key when there are sharps: In all cases, look at the last sharp. Find your key at the half tone above it.
That is usually very uncomplicated. For the key signature with one sharp, for instance, it is f# you are looking at. Move up a half tone, and you find the tonic note of g. You are in the key of G.
However, with your example, the last sharp will be b#, an unusual way to note a c natural, since there is no black key between b and c. Hang in there.
With keys with flats, it is easier. After you learn that F major is the key with one flat, for all other keys with flats, observe the second from last of the flat notes - that is your key.
A key with three flats will show flat symbols on the b, e and a. The second from last one is e, so you are in the key of Eb.
Learning the sequence of flats and sharps is a good idea. Like learning the lines and spaces, there are a number of rhymes and tricks you can memorize to help. I have forgotten them all, but remember that the sharps are added, starting with f, in intervals of a fifth. (f, c, g, d, a, etc.) The first four flats spell the word "bead."
I am sure others here have better tricks to learn this stuff. Hats off to your self- learning. You are following in the footsteps of many fine musicians.