Since it's your first brisket I'd start with something traditional, and then vary that (widely) according to what you like. A typical ratio for a traditional BBQ rub would be 1 to 2 parts sugar (usually brown), 1 part salt (coarse), 1 part powdered pepper (traditionally paprika), and 1 part mixed savory/spicy (i.e. garlic powder, mustard, onion, chili powder). I like my rubs a little less sweet (especially for beef) and usually just go w/1 part sugar. I also avoid buying generic "chili powder" and buy specific types of powder - guajillo, ancho, chipotle, de arbol, habenero, etc. It gives you a lot more control over the flavor, especially if you're going to swap out chili powder for the paprika.
Here's a typical one that I've made that leans more towards chile powders:
1/4 cup brown sugar
1/4 cup coarse salt (use veal salt if you have it)
2 Tbsp Guajillo
2 Tbsp Paprika
1 Tbsp Ancho
1 tsp Habenero
1 tsp Cumin
1 tsp mustard
To my tastes this is has a medium hotness, it you want less spicy you could dial back the Habenero and the Mustard - the other chiles don't have a huge amount of kick to them.
You can really vary rub recipes widely, and use all sorts of ingredients. They'll work for any meat, but I usually go sweeter for pork, more savory for chicken, and spicier for beef. Anything can work though. For random ingredients I've had good luck with coffee, toasted and crushed sesame seeds, crushed pine nuts, and sumac. You can also make a slather and use things like soda, honey or date molasses.
Good luck with your brisket!