This is not a DIY job unless you're a qualified electrician. There are WAY TOO MANY 'issues' that you (and I) don't know that may end up botching up this DIY job, or even killing you. I kid you not. What little I know about something I 'cannot see' bothers me a bit. You should see me add a breaker! My wife makes sure our life insurance is up to date!!! (And I know enough to be dangerous too.)
Get some references from friends/family in the Trades. Get a few estimates and find out why 1 is better than 2 and so on. Don't simply go with the cheapest price for the sake of saving $$$.
And for some unsolicited info, get a 40-slot panel. Upgrade your service to 200 amp (if you can). Have the electrician install a GFCI outlet right near the new box, and have it on its own breaker. Consider an outdoor, weather-rated timer box (for those holiday light displays) on a dedicated 40 amp circuit. If you upgrade, the chances are very good that you'll need a new Electric Meter. Consider installing a whole-house surge protector.