According to you're datatyping, you can put any "String" or char array in there.
Change (or overload) you're constructor to accept a String parameter to which you can assign to this.reverse. According to you're current constructor, you're just missing that parameter signature.
public StringUtils(String reverse)
this.reverse = reverse
// Call with
StringUtils s = new StringUtils("This is my string");
I'm not really sure what the purpose of you're reverse function is though, and it will also give you a circular problem (much like you're previous thread, you're calling a function within a function but not controlling a breakdown for it).
(This smells like homework :P, so no additional code sorry, tis against our rules)
To reverse you're string (manually), iterate through each of you're values from right to left (<string>.length), and assign the values to a new char array of size string.length -1 from 0...<string>.length - 1. So, that the last character in you're given string goes into the first value of you're char array. Return you're char array as a string. <string>.charAt gets a character and a specific position in a string.