Look on this picture http://musicianinyou.com/resources/Piano%20Keys.GIF
To the left, G natural. To the right, A natural (on a piano keyboard).
It would be the key of G. Tubas are C instruments, like a piano.
A half step above G. G sharp and A flat are one in the same.
Yes, A flat is the same black key as G sharp on the piano. It is the middle black key in the three black key groups.
A sharp on the piano could be black notes or white ones. A sharp is note that is half a step above the natural note. A natural note is any of the white notes on your piano. For example if you played the G key on your piano you would be playing G natural. If you played the white note on the right of G you would be playing A. If you played the black note on the right of G in between G and A you would be playing G sharp or A flat. Hope that answers your question a sharp is half a step above a note and any of the black notes on the piano are one notes sharp and another notes flat. for example, E sharp would be F natural, a white key.
Ab
A flat
To the right.
If you look at the black keys they are grouped in 2's and 3's. Now look at the group of 3. F-sharp is the black key in the group of 3 that is farthest left.
The key of E major and its relative minor, c sharp minor, both have four sharps. Additionally, key signatures apply to all music not just the piano and remain the same regardless of instrument. When referring to the piano the term "key" usually refers to on of the actual white or black keys that are pressed down to make a sound of the corresponding pitch. Each key on the piano is attached to a hammer that hits a string inside the instrument, thus the piano's classification as a percussion instrument. The strings are of different lengths and thicknesses, creating the different pitches. Each key on the piano is responsible for one pitch. Therefore a single key on the piano cannot "have four sharps", as you asked.
You play F, C, G, D, A, and E as sharps whenever they appear in the song you are playing.