well,i would think of something that means something to you,eg a girl or something,or think of a hypothetical situation and just think about feelings and key points of it,look at the situation from different views and then write down key words or sentences and try to work from those,eg,i wrote a song about breaking up with a girl,but its seven years after the break up so i wrote the after effects,heres a few lines,
Seven years have passed,
The pictures slowly fade,
We never had a chance,
The memories become vague,
Sleeping on the floor,
You've had better days,
You hope for something more,
For the feelings now mislaid
theyre not the best lyrics sure but once you get them you can work on them changing words etc.its about the memory of me fading away in her mind.as for the guitar,just keep on practicing and eventually itll come naturally.