Convert string to int

Question

I am new at this forum and also in c#, I am developing a windows based form and insert a textbox to take totalmarks input and button to display result according to the given input.

I am facing problem, when I am assigning textbox value to my local variable, my code is
 int totalmarks =  textbox1.text ;

Kindly resolve this issue.

Also I have a question
1- How can I change datatype of textbox to my required datatype like is there any property in which we define its datatype?