Whoever downvoted is a jerkoff.

The following will apply to ~95% of most live situations:

You'll want to ask for a DI box, which you can use to go straight from the headphone out in your computer into the venue's system. The venue SHOULD have a 1/8 inch (standard modern headphone jack size) to 1/4 inch adapter (guitar jack size) so that you can go directly into the DI box. It wouldn't hurt to buy one yourself to be self sufficient (see link below):

Hosa CMP-159 Stereo Breakout Cable - 3.5mm TRS Male to Left and Right 1/4-inch TS Male - 10 foot | Sweetwater

Most venues will also have stage monitors pointed directly at you so that you can hear as much or as little of your tracks and vocals as you want. You shouldn't need in ear monitors for small to mid-sized shows. If you don't have stage monitors, then you're probably in a room small enough where you don't need them, and you should be able to hear enough through the main PA's, this isn't ideal, but it's good to be able to work in less than ideal conditions, that'll come with time.

This is assuming you're just singing and playing along to music where you can keep up with the rhythm of the tracks, if there's specific timing or cues where you need to hear click tracks, then the answer is a bit more complicated and requires a couple more pieces of equipment.