What does "Use of unassigned local variable" mean?

Change your declarations to this:

double lateFee = 0.0;
double monthlyCharge = 0.0;
double annualRate = 0.0;

The error is caused because there is at least one path through your code where these variables end up not getting set to anything.

The compiler isn't smart enough to know that at least one of your if blocks will be executed. Therefore, it doesn't see that variables like annualRate will be assigned no matter what. Here's how you can make the compiler understand:

if (creditPlan == "0")
{
    // ...
}
else if (creditPlan == "1")
{
    // ...
}
else if (creditPlan == "2")
{
    // ...
}
else
{
    // ...
}

The compiler knows that with an if/else block, one of the blocks is guaranteed to be executed, and therefore if you're assigning the variable in all of the blocks, it won't give the compiler error.

By the way, you can also use a switch statement instead of ifs to maybe make your code cleaner.

Because if none of the if statements evaluate to true then the local variable will be unassigned. Throw an else statement in there and assign some values to those variables in case the if statements don't evaluate to true. Post back here if that doesn't make the error go away.

Your other option is to initialize the variables to some default value when you declare them at the beginning of your code.