I got an old el cheapo detector as a gag gift.
How do I go about using it effectively?
The easiest way to learn is to watch a few youtube videos on that particular machine. But I can tell you right off the bat, don't expect very much out of it, It is a very basic detector and likely won't pick up anything more than a few inches deep.
Watch a few videos of how it works, take it in your backyard and give it a go. You can buy a decent beginner machine for a few hundred bucks.
There are also many facebook groups dedicated to metal detecting. Let me know if you need anything else.