For an NSString you would use:
NSString *myString = [theTextField stringValue];
For an int you would use:
int myInt = [theTextField intValue];
There are many other methods for getting the value from a control. Have a look at the NSControl reference for more info, under the “Getting and Setting the Control’s Value” section.
Here’s a list:
doubleValuefloatValueintValueintegerValueobjectValuestringValueattributedStringValue