Strings can be concatenated. In other words you can add Strings with a + sign. e.g., String foo = "foo" + "bar" will result in "foobar". With this knowledge and a for loop, I'll bet you can figure this out.
Note, in the future there will be times when you'll want to use a StringBuilder to do this sort of concatenation, since it is more efficient at it, but for your simple needs, the benefit is less than minimal and so I suggest you keep things as simple as possible and just use plain String concatenation.