I have a 40 gallon hotpoint gas hot water heater in a foreclosure I bought that set for about a year empty. I had hot water pouring out a line under the house so I capped it (stupid I know). Well after talking to a home inspector he told me that it was probably my pressure relief line and my t and p valve was probably bad so I replaced it and yet still hot water is shooting out the line when I turn my water on. For a few hours the water lines in my house just shook really bad and then the valve opened up. I have no idea what to do now. Do I need to buy a new hot water heater or is something else going on?

I must not be understanding. Sounds like you are saying that you had a leaking water line and some one told you it might be your T&P valve and you replace it.
Where was the leak? Where is the water heater. T&P's usually empty into or close to a floor drain.

Did you fill the water heater before turning it on? Sound like you did not and the little bit of water in turned to steam before the thermostat shut it off.

When filling a water heater you have to open a hot water valve to let the air in the tank out before lighting.

T & P valves open up for two reasons so what's the PSI entering your house and what temperature setting on your thermostat? AS far as the pipes shaking and making noise as you fired up the heater that's explained by poor maintenance .
If you do not flush,(not drain) your heater on a regular basis it will build up mineral on the bottom of the tank. When the flame comes on it turns the water trapped under the mineral to steam and the steam escaping is what you hear. Let me show you how to flush your heater. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) and some white grains,(lime or calcium carbonate). The whole thing shouldn't take more then a few minutes. Don't forget to flush it out every month. Your heater will thank you for it. You might want to consider cleaning the screens on your aerators if you have a high mineral buildup. If the problem still persists after you have flushed, you will have to drain the heater, remove the boiler drain and go up in the bottom of the tank with a wood or plastic rod, (not metal) and try to break up the mineral and flush it out. Check the house pressure and temperature setting. Let me know what you find. Good luck tom

I believe there are 2 issues, high house pressure and sediment in the tank. If you replaced the TPV, you should also check the regulated water pressure. Get a teast gauge and connect it to a REGULATED hose bib or on the HW heater, pressure should be in the 50#range. If much higher, find the house pressure regulator and with a faucet open, slowly turn the top stem to the left, loosening it. Note if the water pressure drops. If not, you should probably consider changing the regulator. It may or may not be a DIY job since some are pipe threaded and fairly easy to change while others are sweated in. The HW rumbling is possibly caused bu a layer of sediment and the heated water actually bubbles through the sediment. Note other posts have given you directions to power flush the heater.