If you signed the lease agreement you have a lease agreement with the rental company. Most likely both you and your roomate are responsible for the lease. The easiest way out is to explain to your roomate that you want out and see if your roomate can find someone to take over your obligations under the lease. You woudl need permission from the landlord to get the new person on the lease and you off the lease, or the lease may provide for sub-leasing the premises - which would allow you to rent your portion to someone else - but you remain liable under the lease agreement if they do not pay. Getting off the lease is always the preferred method. Otherwise you are still obligated under the lease and if the payments do not get made the landlord may come after you.
You can always leave without anyone's permission - but as there would likely be finanical repcurcissions if you did that your best bet is to attempt to work it out. Please know that the landlord is not under an obligation to accept a new tenant. Good luck.
This information is provided for general informational purposes and is not intended as legal advice. An attorney licensed in your jurisdiction can answer questions specific to your specific fact situation and provide you appropriate advice as necessary based on the specific facts of your matter and the jurisdiction in which you reside. If you are in Arizona and interested in discussing your matter further I can be reached at: (480) 838-9000 Mark D. Fullerton, P.C. 1839 S. Alma School Road, Suite 275 Mesa, Arizona 85210
I agree with Mr. Campbell in part, but the first step is reviewing the lease itself to see what liability the lease poses as a result of a breach. However, generally speaking, most competently written leases will state something similar to Mr. Campbell's answer.
This answer does not create an attorney-client relationship. This answer should not be construed as legal advice but is given for informational purposes only.