Tracking Aim As A Heavy [closed]

As a player going up against a Heavy, I find myself getting shredded to bits within seconds.

As a Heavy going up against another player, I more often find myself completely unable to take down anything with my mini-gun or lock down any areas consistently.

I realize that aiming for the body helps, as per this question, but is there a better way to track an enemy's movement than "just aim right"? Is my somewhat-outdated computer (slight lag issues) causing me to be a less effective Heavy? Or am I just engaging at the wrong range due to damage falloff?

Edit:

The original question was about how to better track my aim against targets, which is a bit too broad and doesn't really have an answer, though some of the answers given offered good advice.

Shifting this question a bit, I think my problem may actually be related to target priority.

In other words, who should I be aiming for as a Heavy?

Otherwise...I think this question might be inappropriate, and possibly should be closed.

Lag shouldn't be an issue, unless by lag you mean an outrageously bad frame rate.

Miniguns and other bullet weapons are hitscan weapons. They are named this because they "scan" to see if they hit. When you fire, the game instantly draws a line from your position directly forward (or at an angle, since the minigun is not 100% accurate). If it collides with an enemy, they are hit and damaged.

Hitscan weapons use your view of where an enemy is, not where they actually are. This means you do not have to lead your enemies- you aim where they appear on your screen. Leading an enemy can be useful if you have poor reflexes but believe you can anticipate them, however. It's also worth noting that the minigun receives no bonus for headshots. Aim for the enemy's chest- you'll maximise your hits that way.

Distance is extremely important for a heavy. Almost all weapons in TF2 suffer from "damage fall-off". This means weapons deal less damage the further away you are. In addition, the minigun is inaccurate and will miss many of its shots at long range. If you find yourself only dealing 5-10 damage per hit (if you haven't got damage text and sounds on, turn them on) stop spinning your gun and either try to close in or force the enemy to get close to you. Treat the minigun as a rapid fire shotgun.

Finally, be nice to your medic and give him sandviches when he asks. There's a reason the medic-heavy duo is iconic to TF2.