So... you're having some bother sending a text message? But can receive them without a bother? Then something just ain't right!
First thing would be, have you bought your membership? You've topped up you're saying? That's not always enough. You'll need to convert that to membership so have a look herefor more info on that.
So if you've already bought your membership then it has to be something else.
Next thing to do would be to try and check your message centre number - it should be +353894300000for 48. (And it's really important that it starts +353 and not 00353)
Now where do you check it you might be asking? It's different for every handset. Normally, it's in the Message menu and you'll be looking for "Message Centre Number" - sometimes shortened to SMSC! If you're really stuck Google it!
And if your message centre number is right, you'll need to get in touch with us care crew agents here.