Understanding the circle of 5ths is key to being able to play in any major key and understanding if a key is made up of sharps or flats.
Some sayings that I use to remember the orders include:
Go Down And Eat Bread F#ather C#hristmas
Fred Bblogs Ebats Abll Dbogs Gbet Cbracking
The Circle of 5ths:
The Circle of 5ths explained: