Taking lessons and actually learning to play the instrument might help. (Sorry, you didnt' actually say that you already know how to play an instrument)
Make up something original on your instrument, memorize it so you can play it the exact same way 50 times in a row, then record it. (use a click track for timing)
Then play it back and switch to a different instrument and play along with the recording until you figure out something that works together. Usually that is a bass line, but I'm starting to do my drum tracks second lately.
I always make up a song on the guitar or keyboard first, but want to try something new and make up a bass line first this time. Then record that instrument, then add drums, a rhythm instrument (guitar or piano) and a bass line. Ultimately, the drums + bass will dictate the feel of the song.
In other words, create the rhythm section first, then write a vocal part. Some people write vocals first, or write the main instrument and the vocal part at the same time. But for recording, record the vocals and the solos last. I use a scratch vocal part for reference so my drums and bass lines dont step on the vocals, but then the scratch track gets deleted later and the vocals get re-recorded over the finished rhythm section.