You declare a String with double quotes:
String greeting = "Hello";
String name = "Alice";
Concatenation means joining strings together. Use the + operator:
String message = greeting + ", " + name + "!";
// message is "Hello, Alice!"
You can also concatenate strings with other types. Java automatically converts the non-string value to text:
int age = 25;
String info = "Age: " + age;
// info is "Age: 25"
Watch the order of operations. "Score: " + 3 + 4 gives "Score: 34", not "Score: 7". Java processes left to right. Once it hits a String, the + means concatenation from that point on. To get 7, use parentheses: "Score: " + (3 + 4).