There are two types of land. Main land and private estate.
When you have premium account, you can own 512 sqm of main land free of charge. If you want more, you have to pay monthly tier to LL. To acquire mainland, use your search icon on your viewer. There should be a tab called " Land Sale".
Then you get to the land, if you like it, you can buy it at some selling amount. If the land is more than 512 sqm, LL will charge you monthly tier.
Another type of land, private estates, is land that someone rent from LL and sublease to you. You don't have to buy premium account to own land. You can use the land sale search to get to the land, and buy land. The difference is that you will pay monthly tier to a rent box, which goes to the land owner.
What's your plan for the land ? do you want your own island or just a piece of flat green ?
a 512 sqm land is 16x32m and support 117 prims. One full sim is 256x256m or 65536 sqm, supporting 15000 prims (Full prims sim) or 3750 prims (Homestead sim)