The first thing to do is to get a copy of that lease. Start with the landlord, who probably has a copy (and state law may require him to give you a copy if you request it). If you're on the lease, your roommate cannot get you out -- only landlords can evict tenants.

Now, suppose you're not on the lease. All is not lost. If the landlord has treated you as a tenant -- by accepting rent directly, for example, or putting your name on the mailbox or doorbell -- you may have attained the status of a tenant. Surely, if you and the roommate rented the place together and it was clear that you were on equal footing, you should be given the status of a tenant.

Once you are a tenant, a co-tenant such as your roommate cannot evict you. Only the landlord can do that.