In Karthan Ridge, each team has three objectives each that must be defended from enemy attack. In order to destroy the opposition's objective, you must take control of one of two sourcestones in the center of the map and carry it to the enemy objective. The winning team is the one that has the most of the containers at the end of the match.
From the start in Karthan Ridge, you need to rush towards the center and try to capture the sourcestone. The sourcestone is placed on either side of the large center arch, so you can cut off line of sight like the Black Garden match to pick them up. If the enemy team picks it up, then you should aim at killing them so that they would drop it.
If no player picks up the sourcestone, then it would reset back to the center. This is the same if you manage to destroy an objective.
On the subject of objectives, you have three objectives on each side of the map, blue for the Guardian team and red for the Defiant team. If a player who has the sourcestone tags the objective, and completes a 3 second cast time on it, they can destroy it. This cast time can be interrupted with damage.
But if you have the sourcestone, then the best practice is to have both on your side and move as a group towards the enemy base. Your aim should be to destroy the center enemy objective first, since that will be the hardest one to reach to towards mid and end game.
When you do destroy the objective, then you should have someone run back to pick it up again, and then loop back to join the rest of the group.
For this match, support adds an advantage against the enemy. With things like boosting the damage dealt by 15%, or by giving all your teammates a shield, or even removing curses and debuffs placed by the enemy, it is something that should exist in every match.
Aside from that, you shouldn't worry about tanks to hold objectives, but instead should focus on setting up a 1 to 4 ratio of healers to DPS, rounding up. The damage dealers can be either single target or area of effect, but if there are less players you should be on the single target side than AOE.