It is important for you to play with other musicians and the best practice for this is the study of duets. Duets are written as two independent parts of music, which are indicated by Roman numerals at the beginning of each line. To get the most benefit from duets, practice both parts.
The following duet is in the key of G. Remember to play all F notes as F sharp (F#)
This is a natural sign. A natural sign cancels the effect of a sharp or flat for the rest of that bar, or until another sharp or flat sign occurs within that bar. In the second bar of the following example a C# and D# are played. At the end of the same bar a D natural and C natural appear, cancelling the C# and D# sharp.