Very often, there's a seam at the bottom of the bodywork. You'll see a marked location with cutouts or dimples or something. There's one at the front and one at the back for each side. Don't get under any car while it's supported only by the jack. Make sure the jack is on level, solid ground. Be very aware that jacking up a car can be dangerous if you're not careful.
Depending on what type of vehicle it is, first thing you want to be safe. Also there are different types of Jacks. Scissor jacks, bottle jacks, and floor jacks. If you have an owners Manuel, it will show you where it's recommended. The Internet has many self help sites, google it. Use your common sense and think safety.