you are great, you are handsome, you are talented, and you know what, you are a fine pitmaster, yes you are. you can do it all, and it...or, you mean on how to cook a tri-tip.
First off, I really like to find a good Choice grade tri-tip, and if it has a tail, I tie it back, like this...
Makes for a more consistent cook without the little end getting too dried out.
For a rub, you can use one of the commercial ones, such as The Rub Company Santa Maria, Suzy-Q's Santa Maria, Dave and Barney's or you can throw one together. My basic Santa Maria looks like this:
3 parts kosher salt
2 parts medium grind black pepper, no dust
1 part each granulated garlic and granulated onion
1/2 part ground chile, powder from Ancho is nice
1/2 part dehydrated parsley
Just toss it all together in a shaker. You are good to go. Note that there is no paprika or other unusual spices, the flavor profile is pretty straightforward. Personally, I also add 1/16 to 1/8 part of sugar, not for flavor, as that will not be there. But, chemically, it helps the rub.
As for the fire, a lot depends on your pit and comfort zone. On my Kettle, I normally go indirect, 250F or so, until it temps 120F internal. Then I sear. When I feel like paying attention, or am working on a cooker with an adjustable grate, I prefer direct fire, still in the 250F range. This will cook the same time, but, does not need the sear process, so I cook to 135F internal and am done. I rest for 10-20 minutes, tented in foil. I generally shoot for a product like this...
Note that I don't like the outside to be blackened or charred at all, I prefer a more tender, caramelized surface.