To avoid all the Class this and that stuff, just think of controlled (tower with a clearance required) and uncontrolled airports.
If a controlled airport, the runway is assigned by the tower controller, normally based on the favorable wind. Normally, you use the runway assigned by the controller, unless you have a reason to use another one. Thing is, you don't HAVE to do what the controller says if it puts you in danger, and the controller doesn't HAVE to let you go where you want if it screws up his/her flow, and you could say there is always potential for a Mexican standoff situation, in theory. In practice, there will be some mutually agreed resolution, the controller wanting everything to flow smoothly and safely, and the airplane crew wanting to get where they're going without getting violated and without being forced into something dangerous.
So say you are given a clearance to land on runway 9, landing east, but you want to land on runway 18, landing south. So you ask for 18 and see if you get it. The controller decides whether it will cramp his/her day and says yes or no (probably yes 99% of the time). If no, your choices are to either land on 9, go elsewhere, or declare an emergency, in which case the decks are cleared for you but you may have to justify yourself later. In the big picture, common sense is supposed to prevail.
At uncontrolled airports, it's uncontrolled, so knock yourself out. You land on whatever runway you feel like, keeping in mind certain rules and protocols for uncontrolled aerodromes, like pattern (circuit) rules, IFR arrival announcement requirements etc etc. Say everybody is landing on 9 and you want to land on 27 going the other way, forcing an airplane landing on 9 to take evasive action. Probably you'll just get the finger from someone, but there's a possibility somebody will report you for breaking regulations on traffic etiquette and you will have to answer for it, but there is no ATC around so trouble won't come from them.