Don't know which language you ask for
For C: use itoa() function
e.g. int x = 10;
char y[5];
itoa(x, y, 10);
here 10 in itoa() is the base of number used, the base can be hexadecimal, binary etc.
For Java: There are many ways two main functions used are Integer.toString() and String.valueOf()
e.g. int a = 20;
String b = Integer.toString(a);
//String b = String.valueOf(a);
For Python: just use the str() function
e.g. int x = 10;
print("Hello"+ str(x) + "world");