I need help on writing songs?

I've been playing guitar for 3 years and I just now got into singing, my voice is'nt great but its not bad I can sing on key and stuff. And I'm wanting to start writing my own songs, nothing special just stuff like john frusciante, Daniel johnston, and jeff buckley, just the kinda I can do with just and acoustic guitar. I have been able to write several guitar parts but my problem is I can not write any melodies or lyrics to go with it. My main problem though is lyrics, like I know what I want to write songs about but any lyrics I write I think they suck later. And my melodies all suck to .........so what can I do to get better at writing melodies and lyrics that are not corny or popish but ones that are good? Thank You