To Get Started
Here's a quick point to get you started with something to try.
Sending an SMS can be as easy as sending an email.
That's right. You can actually send your phone a text message via email.
However, you have to know the format for the phone service.
For example, Verizon wireless is the 10 digit number followed by @vtext.com (email@example.com) Here's the official skinny on that: Send text messages using email on a PC | About Verizon
Works The Other Way Too
You can also turn that around and send an email from most txt apps. You just type in the contact as an email address.
How Can This Get You Started?
There are a couple of ways that can help you.
1) First of all, you can start texting to your email address and turn email headers on and then you should be able to see the return address for the txt. When you do then you'll learn the "email" address that you can pull out of the headers so you can return a txt to the person who texted you.
2) Sending an email message is extremely easy in .NET (and probably other technologies too).
I guess the old SmtpClient in .NET is obsolete but you can try the open source MailKit GitHub - jstedfast/MailKit: A cross-platform .NET library for IMAP, POP3, and SMTP.
This will at least get you started.