Credit help!!!

Hi so my credit score was a 720 with only a bank credit card limit 2100 and my car that was 18% APR with 15,000 remainder. I just traded in my car the other day for a new one at 5% but my credit took a couple hits with the hard inquiries.. that being said, I was told and advised by the financial person at the dealer that it would help if I had a revolving account.. I don’t make a great load since most of my money is cash by doing side jobs such as helping babysit etc.. when should I open a revolving account? Is it too soon after my credit just took these hits