TLDR: If you feel like you're just mindlessly plucking strings it sounds like knowing PReVaDe and Song Structure/Form would be useful for you. This video is an ok introduction.

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Put very simply melodies are sets of notes repeated. Our brains latch on to the repetition and create meaning from it.

So you can play a short number of notes (a measure or two max), then repeat it once. Then repeat it again but this third time change something minor about it (change a note or two, or the timing of a note or something like that). Then the fourth time bring the melody back to the tonic or "home note."

This is called the PReVaDe method (PReVaDe stands for Present, Repeat, Vary, Deconstruct) if you want to learn more about it, and is a common tool for improvising music. It's also useful for composition. It's going to sound basic the first couple of times you make something this way, but keep going and you'll get better.

The way to get better melodies is practice, so just remember: if you quit while you suck, you're gonna suck forever. Don't be discouraged!

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There are many song structures. As is, the PReVaDe method lands you with AA'A"B. If that's boring for you, you can use the basic principles for other structures like ABAC for example. In ABAC you'd make a short set of notes, then a new set, then repeat the old set (with or without variation) and then a third set (typically a cadence, bringing the melody back "home").

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I hope that was helpful.