If we put a "flat sign" on the B line at the beginning of the staff, it means that
ALL B notes are to be played as B flat.

Below is an example of a score that has a sharp on the F line.
In this example, all the F notes are played as F sharp.

Below are the key signatures you will see in music that have sharp signs

Below are the key signatures you will see in music that have flat signs

Notice that if there are no sharp or flat signs a the beginning of the staff -

 There is a pattern and method to their construction and there only so many key signatures available.

For now, it is not necessary at this time to try and understand the theory of how the key signature are formed.

It is important to be aware that there are different key signatures and what they mean

When there are no sharps or flats as part of the key signature - you are then playing in the key of C. The Key of C is the easiest key to play because you do not have to think about playing these sharps or flats!