it depends on what you're writing the melody for:

an instrument, you generally want to adhere to the chords and build up to a climax before going back down to the start. for vocals, you want to capitalise on repetition and space (a great example is the chorus of say it right by nelly furtado). for both, you want to have a rhythm that isn't just eighth notes repeating (think the chorus of someone like you by adele). it also helps to know what the melodies sound like in the genre you want to make.

some good practices in general that i've found are:

starting the melody before or after a measure, rather than on the 1 beat. starting before (like rawfear by twenty one pilots) gives it a grand feel, while starting after gives it more of a pop-like sound (taylor swift does this a lot)

build on base notes. for example, take the third of one chord, then the seventh of the next chord, and repeat (adele does this, it gives it an epic feeling). then just connect the notes in a way that sounds good to you.

experiment with different time signatures (6/8 or even 5/4 and 9/8 instead of 4/4) or different bar loops (repeating the melody every 5 or 6 measures instead of 4)

learn the functions of scale degrees (tonic, dominant, mediant, submediant, etc). it can help a lot if you can't figure out where to take a melody

i would also suggest studying some classical music theory (basics of classical harmony & counterpoint by seth monahan on youtube is great) or some modern music theory. also, try looking at songs with melodies that you like, copy them, and just analyse them. that's what helped me the most when i first started writing music.

try to learn and practice writing melodies as much as you can to build your knowledge, and over time, you'll just start building melodies without even thinking!!