Right now we don't know exactly when Season 6 starts in Overwatch but we expect it to be around September.

To begin your competitive journey this season, click on the "Play" button, select "Competitive Play" from the submenu, and you're ready to enter a match. 

We don't expect big changes in Rewards so you should get something like the bellow list:

Division CP
Bronze 100
Silver 200
Gold 400
Platinum 800
Diamond 1200
Master 2000
Grand Master 3000

Top 500 had some major changes too, now you need to stay in top 500 when the season ends to get awesome rewards.
Before that, you just had just to reach the rating and you would've gotten the spray.

And there is also skill rating decay. If your rating is above 3000 you need to play 7 games per week to maintain your rating or you will start to decrease.
When you complete a match, the amount of time until skill rating decay occurs is pushed back by 24 hours, and you can bank up to seven days per week. That means that as long as you manage to play seven games over a one-week period, your skill rating will be safe. You could play one match per day or complete all seven in a single session. It’s up to you.