A nice simple phrase, but there are some useful points. The “oo” in “look” needs to be very short, like “put”, not long, like “food”. The “t” in out can be pronounced fully, or down where your voice is, as shown here. The most important thing is that there’s plenty of stress on “out”, and big intonation, because this is a warning.
We also say “Watch out!”, with the same meaning, warning someone that they may come to harm if not careful, for example, “Look out, there’s a broken step.”