Re: Can't come up with new melodies
A lot of good advise already posted. I'll toss my two cents.
Firstly, I agree with relic there - most often it's best to get a rhythm section (the music that's not bass, but is not your melody either) outlined in some fashion if you haven't any tune already rattling around in the head.
But when it gets to the melody, I'd say use Call/Callback and borrow a rule inspired from counterpoint theory but disregard it as absolute.
Call/Callback is just simply to say that you have have two parts to a phrase in a melody, and the second part of that phrase will attempt to end in line with the first, but begin lower or higher than the first part of the phrase.
So you might see C F E, C A B where F and E are higher than C and A and B are lower than C.
It should sort of abstractly sound like a voice cadences in conversation going up and down, but having a centralized baseline (in this case, C) that their voices hover around.
For the counterpoint part, it's just that back in the baroque period there was an obsession with the mechanistic order of music and the theory was more thought of as an order of law as if one was referring to laws of physics.
So from this environment came the idea that there was a correct approach to ascending and descending a melody. You could only go so far, and only by a certain interval at a time, and if you broke that interval, then you had to pick certain options to make for that radical deviation.
Without going into nutters level of theory discussion (because that's what counterpoint talk requires getting into), it's easy enough to borrow the idea of picking a note to start on, then picking it's 3rd or 6th. Now from that next note, also pick its 3rd or 6th. From there, pick its 3rd, 4th, 6th, or 5th (though probably not the 5th too often), from there, pick its 5th or tonic.
Congratulations, you have a simple melody skeleton, now flush it out with more instances of each of the notes like Lug discussed doing.
You'll notice that I didn't indicate up or down direction above. Typically, move small and not in huge leaps. If you leap to a 6th (or higher) instead of a 3rd, retreat after the 6th downward by either a 6th, or more commonly a 3rd and head quickly on your way to your 5th or tonic.
Why not more 2nds, 4ths and 7ths?
Get down with yo bad self!
Typically those are trickier to use to build progressions with for a lot of folks, so I stick with discussing the 3rds and 6ths angle as did beginner lessons in counterpoint back in the days of Bach.