The rhythm or beat is almost like the metronome of the song if you want a slow beat then setup a slow rhythm in your head that goes with the lyrics.

If you want a medium paced beat then find a similar song and copy it til you get the hang of it.

Fast beats/ rhythms are difficult to do and plan for a specific tune.

I usually use slow & medium paced beats that's what I would suggest to start off with then move up to fast paced beats