Unlike a seemingly common belief in lower ranks, the battle doesn't happen on the tower. It happens ahead and around the tower. As a backliner (assuming so based off of heavy), it's your job to take people out in the beginning of the match (actually it's everyone's job), and you need your better slaying type weapons to move up and take out more direct threats to the tower. From this point, usually you would be riding the tower and taking out longer ranged threats to you and your teammates like rapids and chargers and basically anyone you can see.
Don't waste your specials. If you use heavy deco, wait to use those bubbles until you're in a closed off choke area to give yourself the upper hand and move through a checkpoint. Only use stingray on the tower if your teammates are already ahead of you and you really think it would help to move the tower along. It's really not a bad thing to use stingray offensively like this I do it all the time but make sure you're safe and that using stingray would make getting kills easier at that point in time. As for any other time, try to save stingray until the enemy team is starting to push, or if you can tell your team is struggling to fight in mid, then go ahead and pop it to get some picks. I'm a little hesitant to say this because the heavy already offers a lot of nice long range pressure and painting. I don't main heavy, but I know how it works for the most part. This is all I have to say. Get kills.