The top 4 leagues are the Premier League (England) , La Liga (Spain) , Bundesliga (Germany) , and Serie A (Italy) . I would just start watching some games and see which team you start to like, then following them. Premier League this season is all but decided already but the other 3 leagues have been tight races so far.
Other than that, the UEFA Champions League is a tournament where the top teams in Europe play each other and winning it is the main goal for many of them. Overall, this is where the highest quality football is played. The Round of 16 (playoffs) begins in February
There is also international football, where players from clubs get called up to represent their country. Each region has its own tournament every few years. This year there will be the Copa America (South America) and the Euros (Europe). The World Cup is every 4 years and the next one won't be until 2022, but only 32 teams will qualify via qualifications games that will take place over the next 2 years.
And as someone else pointed out, there is also an American Soccer thread if you're more interested in MLS and the US National Teams. Hope this helps!
Edit: This video is very helpful.