"We decided to rent the house and need to know how we go about setting up the trust to do this."
First of all, I'm not sure who "we" are, but I'll assume either that "we" are the trustees of the trust or that your use of "we" means that the trustee(s) is/are administering the trust in consultation with the beneficiaries. As for "setting up the trust to do this," there probably isn't anything to do. Absent some provision in the trust that might prohibit it, there's no reason why a trust cannot rent real property that it owns.
"Do we use the tax id number from my mother or do we need to apply for another one?"
Ugh...that you're asking this question four years after your mother died probably means that the trustee(s) has/have serious tax problems on his/her/their hand(s). A tax ID number probably should have been obtained shortly after your mother's death, and the trust is probably now going to have to file three or four years of back tax returns, which may result in some serious tax liability for the trust, the trustee(s), and/or the beneficiaries. The trustee(s) need(s) to consult with a trust/tax attorney ASAP.
"There isn't any money to hire a lawyer until we have money coming in from renting the house."
Well...the trustee(s) is/are free to take the risk of not consulting with an appropriate professional at this time, but my opinion is that it would be completely stupid not to do so. If the trustee(s) must use his/her/their personal funds for this purpose, such is the consequence of making mistakes like this. Sorry.