First of all, Concrete is the mixture of various sized aggregate (sand and gravel) with (portland) cement (the "glue" that holds it all together) Portland cement is not named after the cities in Oregon or Maine, but after the place where it was invented, in England.
Enough of semantics.
A 25x25 foot slab will need some control joints to deal with expansion and contraction due to temperature changes and drying issues. A good division would be at 5 feet, or 25 squares of concrete.
Depending upon where you live, and the ground conditions, you might consider a thickened edge on your patio, or even a grade foundation, although unless you are putting something really heavy on the patio, this shouldn't generally be a problem. You will want to excavate your site as level as possible, and avoid disturbing any soil that you will want under the slab itself. (Only excavate the soil you don't want there, don't try to put a bunch back because it won't be properly compressed soil.)
The depth of your slab is going to depend on the freeze-thaw cycle in your area, wherever that is. If you have a lot of frost heave, you might consider trying to make sure you don't allow the soil under the slab to get wet, where freezing would cause the patio to heave.
A 3 inch layer of 1-2 inch crushed rock is a good idea beneath the slab, followed by a supported 4" grid of wire mesh (reinforcement) for the slab. If you hire someone else to do the flatwork for you, They will probably place 2 control joints, making 4 12foot squares. However, you will probably do better using pressure treated 2x4s and doing the 5 foot boxes, pouring and screeding (scraping the extra concrete off and leveling it out) each box one at a time.
Don't mix the concrete on your own. Have a truck deliver the premix, because that way the concrete will all have the same mix, and end up pretty much the same color. If you mix it yourself, you won't be consistent with your water mix, so some pours will be light in color, (less water, stronger pour) and some will be darker (more water, and a weaker pour). (I wouldn't try to mix this myself, and I know what I'm doing...)
Your slabs should be at least 3.5 inches thick. At 4", a 1'x1' slab will equal 1/3 of a cubic foot of concrete. 25x25=625 square feet 625sf / 3 (4"=1/3 of a foot) = 208.3333 cubic feet. 208.333333/27 (27 cubic feet = 1 cubic yard) = 7.71, or 8 "yards" of concrete.
To order that much, let them know exactly what you are doing, that you want small aggregate (like 3/4" and smaller) for flat finish work or specify exposed agregate if you want a finish with a lot of little rocks exposed.
Like bc said, it really sounds like you don't know enough to do this without it turning out like a homeowner special (this does not increase your home's value in many cases). I would recommend you hire a flatwork contractor to do this for you if you would like it to look good when you are done.
One last thing, your patio will have 625 sf of surface area to collect rainwater. Consider sloping it to the middle and adding a drain (BEFORE you pour the patio, install a storm drainline) or crown the patio so the water runs off to a point you want it to, hopefully not towards your house. You will also want to run any sprinkler supply lines and electrical lines for outdoor lighting and stuff if you are even dreaming about doing these things later on. It is 100 times easier to do these things now.