First, build your task with your response message. You'll use Send SMS (found under Phone > Send SMS) to send the message. Fit the phone number, you'll specify the variable %SMSRF.
Next, you'll need to set up a profile that monitors incoming texts. You'll choose the event Phone > Received Text. You can specify which senders to act on (or not). The key here is specifying the income message (content). Only one content statement is allowed power task, but there are at least a couple of different ways to handle this (I'll explain in a moment). Next, you'll need to select the task you just created.
Done.
Handling incoming messages...
You could leave the content blank and the profile would execute your selected task. In the task itself, you could examine the message for your required content. For example, if the incoming text reads as 1234, then send a reply text to the sender "Winner Winner Chicken Dinner!"
You could specify multiple profiles, each with a unique code which calls a different task.
Now, be careful. Here's why: If you receive multiple messages close together, things could get confusing as Tasker only recognizes the phone number of the last text message received. It's possible only the last sender will receive all the responses if you happen to get a bunch of texts on top of each other. Tasker dies not keep a log of these text numbers, so the value of that variable I gave you above will be replaced with the latest incoming SMS sender's phone number.
Good luck!