This is a very broad question that can't really be answered with a specific answer. I would recommend you start looking for different options to figure out what you want. For example, what type of phone do you want to target (iPhone, Android, Windows Phone 7)? Map out what you want and then start learning about each step.
For example, here is one option:
http://johnpapa.net/you-are-already-a-windows-phone-developer[
^]
Update
Based upon the new information you explained, I have a better understanding of what you want to do. To allow a phone to text to a service and get back data is actually quite simple anymore. My favorite way of doing this is through a service called Twilio (
http://www.twilio.com/[
^]). You can send or receive text messages for one cent (US) each. They offer an API that is very simple to use and can be easily accessed by C#. Further, they have examples of how to do so and their support staff will work with you, even if you are just testing their service out. I was very pleased with my experience talking to them. I stumbled across a bug and it was fixed within a couple days. Maybe I just hit the end of a release cycle, but either way their customer service was excellent.
If you go this way, basically you just need to parse an incoming text (have key words, etc.) and then run your backend code to handle that request (get airline data, etc.) and then have your code send out a response via text message.