I hate to disappoint Jay but here goes. First I recommend you hire it done which is the easy way as you requested unless you are experienced with concrete which I take you are not by your question.
Otherwise, level your area. Dig a footing 12" wide down to a few inches below frost depth and centered up on your wall. The center line will be 18'3" by 18'3". Pour your footing with concrete so it is level and about even with the ground. Put in 5/8" rebar at every corner and about every 3 feet or so so they go to the bottom of the footing and stick up about 14" to 16".
There are a couple different ways to do the rest but one wastes plywood for forms. Frame up the floor with 2by4s so you have about a 4" floor to pour. dig the floor down so you can put it on a few inches of sand or gravel. You can pour the floor separately or with the wall. Remember you want a 6" thick wall and 18' by 18' is the inside size.
Go buy about 7 sheets of of 1/2" osb plywood and a crap load of 2by4by8s plus some longer ones(you do the math). Rip each plywood into threes lenghtwise so each sheet will give you 3 pieces 8 foot long by 16". Build a series of stud walls 16" tall with a top and botom plate with 2by studs every 24". Nail each plywood piece to each stud wall. To butt two together, drill a couple holes in the studs and bolt them. Remember you need four 2' forms to make 18' on the inside and leave the ends on a couple without a stud so the inside corner can be made. The outside corner pieces will be longer by the double thickness of the 6" wall.
Put 6x6x10 remesh in the floor and run 5/8" rebar all around the perimeter and wire tied to the vertical rebar a few inches above the floor level and a few inches below the 16" inch height.
Install your new forms with plenty of bracing and staked 2by4 diagonal bracing. Drill and use wire ties to hold the forms together at 6" thick. Solid forms and well braced top to bottom will help avoid blowouts which the inexperienced don't know how to deal with.
Have plenty of help and cement finishing tools and call for the truck. Spray forms with diesel so concrete doesn't stick. No problem but still a lot of work even when you are experienced and know what you are doing.
That is enough for now. Go buy a book on concrete work. Home cheapo has a hard bound book by quikcrete for 6+bucks that explains how to do everything in concrete except they talk about using bags of quikcrete instead of having it trucked. I found it laying over by the quikcrete round tubes but it may be in their book section.
This is one of those EPA rules. Think about it on election day. Good luck.