Basic music structure. You have to match "Bar on Bar" and sometimes this can get complicated. Acapellas don't always start "on beat". If you're just starting out, focus on mixing songs that start with a kick and snare that continue through out the song. dance music and hip hop are good choices. Old School (not wicked mix), rock, classic rock are "bad choices". The reason why is because these are real people and the tempo and structure of the music can be a lot more complicated.
EXAMPLE: Commodores - Brick house (drum roll at beginning and then tempo change after bass intro)
Teena Marie - "Square Biz" - starts with vocals then right after the 4 bar intro, the tempo speeds up
EXAMPLE of a Good Choice - Tupac - All about you (starts "on the one" and tempo stays the same)
Calvin Harris - Feels so close (starts on the one and no temp change)