What's the best way to start an exercise routine?

procyonlupus

I'm wondering specifically what's good for my body/age/etc. I've been doing videos online, but how do I know if I'm doing too much/too little? I try to do cardio twice a week for thirty minutes, and strength 3 times a week for 30-40 minutes. I'm 278 pounds and 23 years old, so I'm not sure what's appropriate.