Having land on Second Life is confusing, especially when you get started. I’ll keep this newbie-friendly and give you the BY FAR easiest way to go about it, but you’ll find that I’ve kept a lot of options to myself. (If you want more prims, if you want privacy.)
Should you rent or buy Second Life land?
If you’re happy with less than 1024m2, then rent. Just pay a rent box on a land that is often marked “rent or buy” to get the building rights to that land. You’ll NEED TO join a land group in order to be able to build (rezz objects) on this land.
Another reason why you’d like to rent is that you REALLY like a certain sim and want to live on it, but they don’t sell land. That’s a perfectly pristine reason to rent land if cost is not an issue. In fact, I will be renting houses myself, so if you like it here, rush me… 😉
If you want or need more land than 1024 m2, I’d get a premium account and buy land from the mainland. You’ll have to remember that unless a landowner was born yesterday, they’ll need to cover their expenses, and that’ll come out of your pocket. HOWEVER, groups that own a lot of land get kickbacks from the Lindens, for every full region they own, they get an ADDITIONAL 10% and that land is basically free for them. (In fact, YOU TOO, when you own a piece of land and deed it to a land group, you’ll get that same 10% extra land you can buy elsewhere automatically. Just saying.) That basically free land might be counted into giving more generous prices for their renters, or it might not be.
Buying gives you full control of your land.
If you like computers, you probably also like control over your land. That comes with OWNING the land – even though you’ll still be paying for the land use, basically for a web hosting service in 3D. Just the way you can “own” a website, you’re still “renting” the disk space for it, and the same exact thing happens with Second Life land.
If you’re self-confident about bells and whistles, owning the land will be your best bet.
To own a MAINland parcel, you’ll need:
A Premium gives you a land tier of 1024 m2 or Premium Plus that comes with a 2084 m2 land tier account.
Enough money. (No kidding. 😉 )
Large enough land tier that it covers the land area you intend to buy.
When you click “buy this land” on the land controls (still gives me a rush every time I do it!) the tool will automatically check that you’ve got all three in place. If you don’t it won’t allow you to continue with the purchase. (So sweet.)
How to FIND land to buy
Through Search
Click on Search. 
Go to Web Search -> Land Buy or Rent
Choose your options. Rent prices are shown as N/A, because that field is reserved for sale price. One of the reasons I prefer buying, is so I don’t actually have to go see each and every parcel just to check the price. Lazy.
OR
Go to Land Sales
Under “enter search terms” choose “mainland” (my recommendation, least hassle.) Now the search options are a little lacking, give you that but you’ll make it work. Unfortunately, the land tier operates the opposite way than what is sensible: “find land BIGGER than” not “smaller than” as would make sense. That’s how it is and it’s unlikely they’ll change that soon. Therefore, I often use the maximum price tab instead and limit searches by maturity rating.
Then, I like to sort things with the square meter price, but tell you the truth, the lower the square meters, the crappier the land, and there’s usually NOT ALWAYS, a clear reason why the land sells for cheap just like in real life.
Adult land is FAR FAR FAR more expensive than General land, and Moderate is somewhere in between. If you are creating an adult venue, you might want to reconsider renting from a private land or even buying a private island. However, to begin with, you might want to experiment with an Adult parcel on Mainland to get you comfortable. (You can always build into skyboxes when the ground lever scenery isn’t to your liking, but Zindra (adult mainland) skies tend to be quite full of skyboxes, you might want to fly up to check what it looks like up there before buying.)
Buying a private island
Now if you’re into it, you can actually buy a private island directly from Linden Labs, but a LOT OF PEOPLE sell private islands that are already there. If you buy from Linden Labs, the island is created for you, you’ll become the Estate Owner, and later, you can add islands to your estate, either land attached or detached.
Check current private sim pricing.
The “price” refers to basically a setup fee + first monthly fee. You’ll be paying the lower monthly fee starting from the next billing cycle.
Full region – allows commercial use, 20 000 prims (and an additional 10 000 prims for a separate fee).
Homestead – allows residential use only and holds 8 000 prims.
The skilled gaming region is basically a legal gambling region. (I really don’t know the legal requirements of owning one, so you’ll have to google that if you’re interested. Why it’s called “skilled” I have no idea.)
The private island is about the size of a city block if you honor the metric scale that everything on Second Life is based on. It’s a bit bigger than in New York, which is built very tight. Still, people tend to feel comfortable building shop outlets the size of a city block without blinking, so… There’s a lot of space for size anarchy there.
You can choose a starting landform of a Full Region island here. After you buy it, you can still terraform it into any shape you want – or go from land to only ocean. But first, you’ll need to contact the Lindens to set it up for you. You MIGHT feel going to see a readily modified sim in-world to buy it may feel romantic, but you’ll get the same deal with Estate Owner rights when you go through the Lindens.
In the settings, you can choose if you want to combine this island with a friend’s island IF the friend has opened up their sim for you to do so.
Event sims
Just to tip you off, you might be interested in hosting a Second Life event – there is a way to get a temporary sim for the use of an event, too.