What is the criteria for getting into it?
The Champions league is a football tournament organised by Union of European Football Associations(UEFA).
Read this article for learning about the format and criteria for playing in the UEFA champions league.
The criteria is a detailed one, which is best explained by reading the wiki link.
How is the season scheduled?
The season starts around July with the qualifying matches being played. Once the qualifying rounds are over, we have 8 groups having 4 teams each giving a total of 32 teams.
Then the group stages begin, followed by round of 16, quarter-finals, semi-finals and then finals.
The dates for this year's champions league are:
- 1 July – 27 August 2014 (qualifying).
- 16 September 2014 – 6 June 2015 (competition proper).
What takes priority (a domestic league fixture, or a Champions League
Champions league matches are generally played on Tuesdays/Wednesdays and the league matches are played over the weekend. So mostly they don't clash. But there have been instances of the league's governing body(FA, LFP), rescheduling their league matches if their league's team has an important champions league match.
Regarding the player's salaries and contracts: It's better if there is a separate question regarding this. But i do believe the salaries and contracts would change when champions league is involved with respect to their salaries and bonuses.