First you gotta know what key your instrument is in. For example the French Horn is in the key of F. Then you gotta have basic skill about notes like quarter notes and half notes etc. Then find a basic program on the computer to write your music.
Dynamics and articulation
Make the song more fun put crescendos and decrescendos to make it soft and loud at different parts instead of the same volume the whole time. Also put articulation like staccato and legato and glissando.
Come up with something original Maybe a Pep Band tunes or a classical song or pop tune.
Make sure your songs is not copyright if you published it you will be fined.
I wrote a song for Pep Band i hope you like it
Finale Note Pad (The program i use)