Some people want to write music. Some people don't have a clue how. But fear not! There is a way! One of the fundamental part in writing music is  Music Theory! The website those two words link to, teaches the very basics of music theory. You can also find plenty of interesting links on that site too.
Here's more on the basics. Then you can also try to find books about it, but unfortunately I can't point to any good ones since all I've been using are untranslated Icelandic books. But you should be able to find books on the matter in stores that sell musical instruments. I do hope this will be of some help.

Sweet! Thank you!

Will it teach me rhythm?? ha ha

Well, the second link has lessons on measures, time signature and note duration, if you haven't checked those out. It's pretty much the basics of rhythm.

Although I'm laughing at Bunny I have to wonder if rhythm can be taught. That's a very helpful resource to have so thanks Djeinus!

There's this fun thing my music teacher made me do when I wasn't getting the rhythm right was to count the time signature (if it was 4/4 I'd count to four, and if it was 3/4 to three, you get the picture) and clap the rhythm of the song. Here´s a wikipedia article about counting. I also have found stepping the time while playing helps.

Oh wow, I learned something new today! I always wondered how that worked, thanks Djeinus!