Someone's shooting at my plane out of the blue! What do I do?
For my answer I'm going to assume that someone is genuinely trying to shoot you down for some reason, it's not some sort of legal intercept. If you ignore a legal intercept then I can see a warning shot being fired to get your attention, in which case I would tell ATC I've been intercepted and get on guard (121.5) and communicate with them and follow their instructions, evasive maneuvering in this case is a bad idea. I'm also assuming that no missiles are involved because if you are against a missile equipped enemy in a PA-28 the missile's going to win. It may be called a Warrior but let's face it, it ain't. 

In the incredibly unlikely event that someone is trying to kill you without warning then communication with the ground would be my last priority in the situation you describe because ATC cannot help you and you don't have time to explain it to them anyway. If you have a one-button squawk emergency button by all means hit it, but first and foremost you need to keep positive control on the airplane. If you exceed your critical AOA and end up in a spin you are in a worse emergency and if you rip the wings off you are dead. 

If it is at night then turning off your lights would be a smart idea, this won't help if the fire is radar guided but it will if the fire is visual. If there's a cloud very close by going into it is probably a great idea. 

Next you need to determine whether the fire is air or ground based. If it's ground based you will know because the tracers will be going from below you to above you (hopefully not through you) then your best bet is to throttle up and clear the area as quickly as you can by staying on the same track so you get away from the guns. Diving will increase your airspeed allowing you to clear the area faster but keep it below the vne! Some small left/right turns may help to throw of the aim of the guns, but large turns will only serve to keep you close to the guns for longer. If there's terrain nearby which you can put between you and the guns then use it. 

If the fire is air based then the tracers will be closer to the horizontal. If you are being shot at from another aircraft then flying in a straight line is likely the worst possible idea, you need to maneuver. If your airplane is faster than theirs then dive and outrun them, but chances are you aren't going to be able to do that, so personally I would corkscrew down and cling to the nap of the earth. Use your short turning radius to its best effect and don't fly a predictable path. Ideally I would use terrain and/or clouds to try and break visual contact so I can escape.  

If you can spare a few cycles in all this you should make a mayday call to ATC and tell them you're being shot at, and whether it is another aircraft or someone on the ground. Say your intentions, your aircraft condition, and the number of people on board. This isn't likely to immediately help you, but it may help to keep someone else from flying into the same situation if there's a nutcase on the loose with an anti-aircraft gun or someone's strapped a 50 cal to their SR-22 and is playing red baron with passing aircraft. If you're lucky local law enforcement or the military may be able to scramble something fast and take the heat off you, but most likely it's all over by the time they get there.