For me the initial chord progression is the hardest first step. Once that is down-- at least the verse part, its easier to get started.
I suggest you don't worry about writing good lyrics. Just spit out anything that comes to the top of your head. My first lyrics are generally throw away things that will be replaced later once I have the melody down.
Often a line will come up that catches my attention and becomes the basis for the song. Writing song lyrics-- at least when I am just beginning a song often seem to be something that pops out of my subconcious. Try not to break the flow by editing as you go.
In short, my advice is just to hum along, eventually something will come. Don't get yourself married to any words until you at least get a verse and a chorus put together. If the song doesn't come all at once, don't beat your head against the wall. Just give it time to gestate and come back to it days (or weeks later.) Once you get the verse and chorus down, you'll likely want to write a bridge-- based on chord progression or vocal melody that is completely different from verses and choruses.
If you get a verse and a chorus done and are having trouble with a additional words, you might consider putting down the guitar and picking up a pen. Additional verses-- the simplest ones anyway-- just follow the same metrical scheme as the original verse. If you don't know what that means. Just count the syllables in every line and try to repeat it in a separate verse.
After you write a few songs you start to come up with a structure you can repeat. For me one typical format is
Verse 1
Chorus 1
Verse2
Chorus 1
Extended Chorus (often a repetition of Chorus 1 with new lyrics.)
Bridge
Verse 3
Chorus1
Extended Chorus
Songs can take a number of forms like this. A short song might be:
Verse1
Chorus
Verse2
Chours
Repeat Chorus
Anway, in a nutshell that's usually my strategy