Do us a favor, post a pic of what you've currently got. You should have a bassett Tee to control water flow to the headers, otherwise you're in for some fun!

Taking. A second look at my setup I do have the t valvesrunning to my water injection lines to the headers. I think I'll start with the water inlet valve on my jet to control the temp because it doesn't get over 120F.

Home depot valve? Can anyone tell me what kinda valve, size etc I should ask or look for? Do I just cut the inlet hose somewhere in the middle and insert the valve, tighten it down with some clamps or should I run a longer hose to the driver seat, valve it there and run it back to the waterpump housing that way I have the control within my reach?

http://www.rexmar.com/images/page228.jpg    this will help you out

Those are the valves I'm currently running on the headers, what about the water inlet valve? What kinda valve should I pick up?

I have pipe right of the jet straight to a ball valve. If for some strange reason you blow a water line, the ball valve will be easier and quicker to close so you don't flood the boat. Some people like a gate valve here so they can get tighter control on the temp. I run the valve 1/2 way open. If I open it too much = milkshake.

I not sure what size the pipe is 3/4" maybe. 
If you do a search on Perfomance Boats, you should find a lot of info.

Thx for the info, I'm gonna get a valve according to the hose size and also run a water pressure gauge so I take out the guess work how much to ipen or close the valve.

This is the water that cools your motor so your engine temp should determine where you set your gauge.
The way I set mine is to back it off until I got about 180*- 190* idling around. Then when you get on it your temp is going to go down to 100* or so. That is just the way it is with this setup. Hopefully, the pressure is reduced enough not to cause any problems.

If pressure is still an issue or concern, then you might want to think about a regulator/bypass deal.

(should be a pic of the bypass system)
http://hi-techperformance.com/HPParts.HTM
(should be a link to the page)

Do I just cut the inlet hose somewhere in the middle and insert the valve

This sounds like you got a rubber hose right of the jet. You might want to get a piece of hard pipe from the jet to the valve.
Like I said earlier, if you blow a water line, you can reach back and shut it off quickly with a ball valve. If you have a hose between the jet and the valve, and that's the one that busts the valve does you no good.
Good luck