LONG POST INCOMING
I have insomina right now and can't sleep so I will try to answer your questions in order with detail and also have tl'dr for each question. Remember OP you asked for this.
1. Is there a season?
There is no season for wrestling. Monday Night Raw, WWE's premiere show, airs every week for the entire year, and every 4-6 weeks a special event, normally known as a pay-per-view, is held that typically ends that cycle of storylines.
TL'DR: No. Wrestling is always on tv, and there are special pay-per-view events
2. Is there a playoff or championships?
There are belts that the wrestlers fight for, there are multiple belts with different levels of esteem placed in them. The current highest rated belt is the WWE World Heavyweight Championship, and the champ defends whenever the storyline says so, but normally it is defended every pay-per-view.
TL'DR: There are title belts but no playoff or championship in the traditional sense.
3. Is it fake and do they acknowledge that?
It is very much fake, matches are predetermined and everyone on screen is playing a character on some level and wrestling fans know that. WWE acknowledges their performers not as wrestlers but as "sports entertainers" which they do to make it clear that everything is scripted. However on the shows themselves they do not break what is known as kayfabe, which is the term for acting in storyline and like your character. the crowd cheers along because it is fun and part of the deal of going to a wrestling show.
TL'DR: It's fake and everyone pretends it is real b/c it's fun.
4. How did it become fake?
This is a long story but essentially people who ran wrestling promotions had legitmate contest but realized that people would rather see a good guy face a bad guy and eventually overcome him then actual sport and made more money with scripted matches.
TL'DR: More money in fake stories then real sport.
5. Is it worldwide and is mexican and Japanese wrestling fake too?
The wrestling styles differ in different countries, but the answers to all parts of this question are yes.
TL'DR: Yup.
6. Is it all one big league?
No. For a long time wrestling was regional and different companies ran different territories without infringing on each others space too much. More popular dudes like Andre The Giant would go and travel around but most people worked in a single territory for most of their career. That is until Vince McMahon Jr. took over his father's company, the WWF, and started taking other territories talents more agressively and also got a TV deal. This is the beginning of the modern idea of wrestling, with WWF (later renamed to WWE) as the biggest wrestling business in the world, but other wrestling promotions and companies still exist and in the 90's a company called WCW actually beat WWF in ratings and was a rival for a long time (WCW was what you were watching when Hogan became a bad guy OP) however, mismanagement lead to WCW going under in the early 2000's, when it was then bought for a comparitively cheap price by the WWE.
Right now worldwide the biggest promotions in each of the major wrestling countries are WWE for the US and overall world wide, New Japan Pro Wrestling (NJPW) for Japan and Asistencia Asesoría y Administración (AAA) for Mexico.
TL'DR: No, there are many seperate promotions worldwide.
7. How do they decide who are good guys and bad guys?
Typically either a wrestler will make a character for himself or the person writing and running the promotion they are in will make a character for them to play. Vince McMahon is infamous in more recent years for heavily scripting the wrestlers promos (the times they talk rather then fight) and overall stifling creatvity from wrestlers themselves by having total creative control on who does what and when. Oftentimes characters will switch roles from good guys to bad guys or vice versa if they are stagnating in popularity in their current role or if the story demands it.
TL'DR: The writers or wrestlers themselves. Specifically Vince Mcmahon in the WWE.
Also, about Goldberg, he eventually did lost and had his last match at wrestlermania tourney against former WWE Champ and former UFC Champ Brock Lesnar.