I'm not sure I'd be able to help, but has SMS worked on this phone in the past and it stopped working or has it never worked?
A soft reset could fix the issue... press and hold the power button and volume down button until the phone restarts.
If that doesn't fix it, it could be a temporary issue with your carrier or possibly an issue with the SIM settings on your phone. If it's convenient, you might stop by a local carrier store and see if they can help with the settings. If not, maybe someone here can help, but would probably need to know what carrier you are using, ATT, Verizon, Etc...