Be consistent in how often you do it. The schedule that seems to work best for me is that I work out a bare minimum of every other day. That means if I worked out yesterday, if I want I can take today off. It also means that if I didn't work out yesterday I must work out today

When actually exercising give it a genuine effort and push yourself but don't overdue it at first. If you are starting from scratch I would only do 10 to 20 minutes at a time, 10 minutes is more than enough. But within a few weeks or a few months at most you should be doing 20 minutes to a half hour at a time, longer if you have the time and desire