Re: is it necessary to clean the gas tank before first run?
A quick look with a flashlight should tell you if there is a lot of rust/crud in the tank. If it looks relatively clean then use Dave's advice. If there appears to be rust on the interior surfaces then dump in a hand full of course sand, lead shot or any abrasive material and give the tank a few minutes of hard shaking then rinse with hot water and allow to completely dry before installing it. Many of the kit supplied tanks, especially the chrome plated ones will have rust inside that you need to get rid of or you'll have problems with clogged fuel filters, needle valves and other problems. Most of us like to use an external fuel filter betwen the tank and carburetor to assure a clean fuel supply to the carb.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"