Check pressure in the system it should be 15-20 lbs. Turn gas line on supplying the boiler and turn on power. make sure water warms pipes going away from boiler and water coming back is cool when you turn on your thermostat.
First you need to check the safeties (low water, high pressure, pop off valves, etc.). Then after you run it enough to check the safeties you should make sure it runs in the right pressure and or temperature ranges. You may want to call a service company for an initial start up if you have never done this before.
