SIMSIM:= "Simulator". This is the technical word for "Island". SL is build upon ten thousands of such islands, each has size 65536 virtual square meters... Many of them are owned directly by LL, this is "mainland" and can be rented. Private sims are often also for renting, but they will typically be more expensive due to understandable reasons. Occasionally you can find very interesting offers for reasonable prices. Just note, that a full sim can be rented for about 300 US$ per month (plus an initial fee of which i believe it is around 700 US$ ) and that includes up to 15000 Prims to be used for building. There is also an offer with reduced number of prims (not sure if it has 7500 or 3750 prims per sim).
So if you can afford that, it is your cheapest option regarding cost per prim and cost per square meter.
You can rent a full SIM from LL within a day or so. You can rent parcels of almost any size (although 512 sqm seems to be a reasonable minumum) from private owners instantly. Many owners have automatic renting systems installed. Just click on the "rent" button and go ahead...
Please note, that there are a few "sand boxes" available. This are SIMS or parcels on a sim, which have been opened for public building. Your creations are volatile though. So better save them in your inventory before you leave the place again

Sandboxes are typically for free usage. The owners might ask for donations which is legitimate as somebody has to pay for anything you see in SL (although it often appears to be an entirely free environment)
BuildingBuilding itself is free. But typically you will get a limited amount of Prims (primitive objects) which you will be allowed to create on your rented land. You will have to take care about the rules and the pricing though. Some land parcels are even rented not on the square meter amount, but on the used amount of prims.
structureThe sructure is as customizable as your fantasy can go. well, not 100% though. There are limitations of course. For instance there are 2 types of objects, which you can create: Prims and Sculpties. While prims can be created instantly in world, sculpties are created outside of SL and their construction is a more tedious process. There are tools around though.
Don't you have any friend, who can guide you online ? It might make sense to arrange an appointment in world... If you like so, we could arrange something. I am typically in world around 1 PM SLT, sometimes i can manage to get in earlier. If you are interested, don't hesitate to IM me in world.