For lyrics, try something that matches your style of music. From your description it seems like you have a Pearl Jam-ish style (correct me if I'm wrong). Thus you would try to write something that tells human stories (think "Jeremy" or "Amongst the Waves"). That similar logic could be applied to other styles, ie if you were an 80's glam rock style band you might consider love song or party-it-up type lyrics. I don't know what your band sounds like so I can't tell you what sort of lyrics you would have to write. As for the actual WRITING part of writing, 1) Don't think you need to make it rhyme, 2) Change up the amount of syllables in a line ("Blow Up the Outside of the World" by Soundgarden is an example), and 3) Most people can't really hear the lyrics so DON'T pour more effort into the words than you do the instrumentation.
As for instruments, first decide what kind of song you'd be writing. Anthem, ballad, acoustic, electric leads, etc. Start with the rhythm as it is easier to build off that then to build off a crazy guitar solo (More specifically, go drums > bass > rhythm guitar > everything else in no particular order). As I'm sure your aware, the lead can do all tons of amazing stuff but the song will still suck if the others fall apart. As for actually coming up with riffs/drum beats/bass lines, improvise with what you know. The pentatonic scales work wonders in all genres of rock.
Keep in mind that while it is important to press yourself, never create a piece of music you can't play well and play consistently. You'll have to play this in front of people at some point, you know.
Also, never let others tell you what you can or cannot do. That's sort of a given.
If all else fails, have the drummer play a basic beat and let the guitarist do some E minor pentatonic blues soloing. Like Jimi.
