Class A is the simplest airspace to understand. There are no weather minimums to remember and there is no map to consult to locate class A airspace areas.
Class A airspace covers the continental United States and Alaska including the water out to 12 nautical miles from shore. The bottom of Class A airspace is at 18,000 feet MSL and the top is all the way up at flight level 600. Very few planes fly above FL600 (currently no airliners), so you can think of this as all of the high altitude airspace.
All aircraft in class A airspace must be operating under instrument flight rules. This is why there are no weather requirements. The communication requirements are equally easy to understand because before reaching 18,000 feet you will almost always already be operating under an instrument flight plan.
If you find yourself VFR below 18,000 feet in an aircraft equipped to reach class A airspace then you will need to call ATC and ask for an instrument clearance so you can begin operating under IFR before climbing.