abc notation is one of the easiest. at its simplest, you just need to specify the note letter and length.

example c d/2 e2 f3

is a C, a d which is half as long, an e twice as long as the c, and a d three time as long.

If you think you can handle that, then there are links here to the abc site with a full description of the notation - but you don't need much more than the above for a simple tune. Here is a link to a tutorial abc tutorial