When it comes to Competitive Overwatch you should be at least lvl. 25 for that is the requirement to start your placement matches. Competitive gives your rank based on how well you did in your ten placement matches.
The Competitive ranks are: Bronze (1-1499 sr), Silver (1500-1999 sr), Gold (2000-2499 sr), Platinum (2500-2999 sr), Diamond (3000-3499 sr) Master (3500-3999 sr), Grandmaster (4000+ sr) and TOP 500 (top 500 players in your region regardless of tier)
Currently, Overwatch is in Comp. season 4, (Comp. = Competitive) and how the ranking goes is let’s say I was in Silver in season 3 (which I was) in season 4 even if i did my best, won all my games, ect. I would still get Bronze- Silver rank. Let’s say In season 1 I was in Top 500, in season 2, I would get Grandmaster- top 500. (None of this is true xD) so basically, even if you win all your placement matches you will still get a rank in the same category as your last season rank regardless of how well you did.
The rounds/games of Comp. are different to quick play. They go bye matches, let’s say we have 2 rounds and we win one and the enemy wins the second, it would go on to round three. It goes all the way to round 5 and if you tie then, it Draws, And both teams kinda win. If you loose, then your SR goes down a little bit, if you win, your SR goes up a little bit and you get a few Competitive points.
Currently in season 4 your SR decays due to inactivity. To stop that, just play once a day. Or you loose your rank.
I hope his helps you with your matches. Also, you usually see teams group up or kill the healers first. And if someone tells you to switch to a rank or healer, do it, you probably need it.
Remember always, stay as a team and protect your healers. This is Smuh again giving you another Overwatch tactic.