practice, practice, practice
thats the best i can offer you
but start with some basic drills to get you going and hopefully develop an understanding.
play a regular a minor chord- a, c, e
hold that for one bar, and loop it for four bars, so you have a soft stab on the first measure of every bar.
find a string patch. now play a basic melody line using notes within that chord. a,c,d,e,g for example. spend a few minutes crafting a basic melody over the piano chord. dont hesitate to use both sides of the scale eithin the melody!
get a basic melody going, and find a bass patch. focus on just the root notes, being a,c, and e again.
no, this isnt rocket science, but its a great starting point and sometimes if i am trying to break a slump i do this exact same thing and it helps open my mind up.
obviously once your chord progresions get more complex and you arent just doing one bar stabs, things get much more complicated when you are arranging..... but start here, apply the same general princeples to your compositions no matter how complex, and you shuld be ok. you just have to work it out, really.