Guest Annie: You need to learn about the circle of fifths. See this article in Wikipedia.

The article tells you way more than you need to know and is full of technical jargon. Never mind that. Just look at the illustration.

In the circle, note that the C is on top, and the chords on each side are G on the right and F on the left. This means that when you play in the key of C, the chords you will use most often are C, G (or G7) and F. The G chord has D and C on each side; therefore, when you play in the key of G, you will use G, D (or D7) and C.

To transpose any song from C to G, you just move every note and every chord one step clockwise around the circle.

That is, C becomes G, G becomes D, a-minor becomes e-minor, and so on.

To transpose from C to D, you move everything two steps around the circle, and so on.

Happy transposing!