With Merengue, you can always find a really slow song, or a really, really fast. If you're a beginner, try to stay away from the fast songs, because it will complicate your life to stay in beat.
Try from the slowest to the middle ones. You'll hear it with the music. The fastest they go, the more instruments sometimes they have, and as fast as it sounds, stay away from it.
Take the ones that are really slow and the beat's clear for you to dance. More important is that you have fun, and every single time that you go on to the dance floor, that you're having fun, that you're actually doing what you have to learn and showing it off.