Get a limit number to test all the prime numbers it contains in C
Personally, I would put FindPrime above main, to avoid having to declare it.
Only have one variable declaration per line. With more than one, it becomes harder to see where a variable comes from. It doesn't really matter with a short program, but will for more complex ones.
Your print statements contain text which contradict what the task says (contains rather than less than)
userInput needs to be validated - Make sure that the user has entered a number, and it's greater than 0.
As mentioned, it's better that a function does one thing, rather than multiple. So your FindPrime function should be responsible for finding one, and the caller should be responsible for printing it.
In FindPrime, the if statement contains a condition that's already been tested for in the for loop, so you can simplify the if condition by removing it.
Always use {} for if (and while, for etc) statements, even if they're single line. Sooner or later, you'll add another line and break things, causing a very subtle error. And in this example, I'd reverse the condition, since the normal case doesn't do anything.

if (number%numberTest != 0 )
{
    nextNumber++;
}



With regards to the algorithm, it can certainly be improved, but that may be out of your reach at the moment.