"I don't think I can or should right (sic) the lyrics."

Then, unless your composer/collaborator is totally in tune with your plot, characters, development etc. to the point where, having delivered the non-musical bits, you're redundant to the project, I'm afraid you may be heading for disaster.

The songs in a musical - certainly in this day and age - are an essential part of characterisation and storytelling. They're not just bits of music stuck into a prose play. Every song should, even must, advance either the narrative, or the characterisation, preferably both. The very fact that a song takes the audience out of the normal dialogue situation, points to the fact that this is a majorly important part of the story, a highpoint, something so significant that spoken dialogue is not enough to convey what needs to be conveyed. Songs can be an emotional short-cut, saying and showing in a few lines what it would take a great deal of lengthy dialogue to express, and without the added emotional benefit which the actual music brings. There are numerous great examples of where a song transcends the normal theatrical narrative and gives a deeper, more impactful insight into characters and their motivations. May I cite - and this is surface skimming off the top of my head:

Don't cry for me, Argentina, I Know Him So Well, Defying Gravity, Mr. Cellophane, Some Enchanted Evening, My Boy Bill, Bring Him Home, Oh What A Beautifil Morning and, even, Let It Go.

To my mind the songs in a musical, especially their lyrics, are an integral part of the whole thing and, I believe, should, preferably, be written by the person responsible for the 'book.' After all, it is he/she who is closest to the story and characters. Of course, it's perfectly possible for someone to write the 'book' of a musical (i.e. the structure, scenes, spoken dialogue etc.) but he/she will always have an eye to where a lyricist and composer will work their magic. Very often, such writers work in close collaboration with the lyricist and composer, exchanging ideas, virtually 'growing' the piece as they work and feeding off each other's expertise. No matter who writes what, the songs in a musical are always paramount, not only for their tunes, but also for their power to tell the story better and with more emotional impact than spoken dialogue.