Removing duplicate chars from a string passed as a parameter

Removing duplicate chars from a string passed as a parameter

I am a little confused how to approach this problem. The userKeyword is
passed as a parameter from a previous section of the code. My task is to
remove any duplicate chars from the inputted keyword(whatever it is). We
have just finished while loops in class so some hints regarding these
would be appreciated.
private String removeDuplicates(String userKeyword){
String first = userKeyword;
int i = 0;