You'll need to provide some more info - e.g.:
Do you have an active membership?
Do you have a text allowance?
Are you trying to send MMS rather than SMS text messages (e.g. picture messages)?
What sort of number are you trying to send a message to?
Does sending to any number work?
When it fails what error message do you get?
Please answer all of these questions so that I or others here can help you.
Hope this helps.
