a few i can mention la...
listen to more songs
be creative
dare to try n experiment
sumtimes u dun realise like wen u r humming a tune or wateva,note it down,ideas will come.yes,u still have to noe how to arrange it well. ex:less is more sumtimes.
u can either do the whole song 1st,then listen to it for a few days,maybe months to get a melody for vox.but sumtimes,u got a beta idea to put in or improvise. it takes time,dun rush.
i have a frd who write the lyrics 1st,then think for a melody. and even noe wat chords to play! went bck hm n just arrange. tat guy got music chip in his head.
and
be patience...