Since the function is log, we know that we cannot take the log (ln in this case) of a negative number or of 0, so the term inside the function must be >0.
Step two: Find these values of x.
The term inside the log is (x + 1), so:
(x + 1) > 0
x > -1
That is your domain, which can be written as:
D = (-1,inf)