If you search around these forums you will find quite a few threads about this, so I'd recommend poking around and doing some reading.
Familiarize yourself with the manual and follow its recommendations for maintenance.
If it didn't come with the newest Militia Lube (tube is newest, jar is older) then I would recommend picking some up.
The GT will run LP & HP but I would recommend against HP unless you have no choice. I personally run SLP and love it.
The pressure tester is nice to have for getting a baseline and for troubleshooting.