...make a rub.
Rub making is probably the easiest thing one can done in the process of making barbecue.  You take a few herbs and spics, chuck them in a bowl, and mash them around to combine.  It just doesn't get any easier than that.  With the exception that you could buy a commercially prepared rub in a bottle.  There are number of these that are quite good and require that you do nothing more than shake them on.  There are quite a few championship barbecue teams that have built their success on rubs like Smoking Guns hot.  Even though they are easy and simple they are very important to the art of barbecue.  Rubs provide almost all the seasoning early on and they are essential to bark formation.  While some chefs will have every spice known to man in their rubs two ingredients are all that is required.  Let's look at those two ingredients and a few others that I find make a delicious rub.

The two magic ingredients-- salt and pepper.  That's it.  There are numerous barbecue shrines in Texas that use only S & P on their briskets and clods.  If you choose to go down the path of making your own rubs this is the best place to start.  Use S & P generously and smoke the meat until it is fall apart tender.  This will be good barbecue, will provide a good base on which to build the other flavors, and allow the flavor of the meat to shine.  No herbs or spices just smoke and seasoned meat.  

I say simple salt and pepper, but that isn't just any salt or pepper.  While one could get away with restaurant grind black pepper it will be better if it is fresh ground and I prefer Tellicherry peppercorns, but Malabar will work nicely too.  Salt, well, all salt is not created equal.  For rubs and such I prefer kosher salt.  Diamond Flake or Morton's?  You choose.  Just be aware that they will have different weights by volume so since I use Morton's that's what my measurements are based on.  If you choose Diamond Flake be prepared to tweak the quantities.  I may sound mean for a minute, trust me I am not, but if you choose to use the iodized table salt that we grew up with do not blame me when you serve your barbecue.  In fact don't even tell anyone you got the recipe from me.  It will not be good and you will be better off to just buy some prepared rub.  Pick one... any one of them will be better.

Leaving the world of simple salt and pepper behind there are unlimited possibilities.  Probably the most ubiquitous ingredient in a dry rub is paprika.  Just plain old paprika.  There isn't any need for expensive Hungarian paprika or smoked paprika or hot paprika.  Buy the big container at a decent restaurant supply store and there will be plenty for several batches of rub.  Some will tell you that its flavorless.  Well... it doesn't have much, but it should taste like more than dust.  It is, after all, essentially ground bell pepper.  I believe it acts as a carrier for the other more pronounced flavors as well as a component of that bark we like so much.

Sugar is next in line and it isn't in every rub, but it is common.  All sugars are not created equally either.  There are several kinds of sugar and any of them will do.  Just be aware that they each have their own characteristics when cooked.  Most sugars are fine with low and slow, but high heat and sugar are not good bed fellows.  A quick blast of high heat can give you the wonderful crackle we expect from Creme Brulee, but any longer will lead to something that tastes a lot like carbon.  Because it is.  My preference for sugar in rubs is now Sugar in the Raw or Natural Brown Sugar.  It is to sugar what kosher is to salt.  Being less refined it doesn't burn or darken as quickly and I think it is a little less sweet as well.

From this point it is up to your palate.  The oldest reported rub is called the Cold Mountain Rub-- equal parts salt, sugar, black pepper, and paprika.  A tablespoon of each will do and can be doubled or tripled as needed.  Almost any other spice you like can be added.  Garlic, onion, and chili powders are very common additions.  As are ground mustard, cumin, and coriander.  Various chili powders such as Ancho or Chipotle can add heat as well as the ever popular Cayenne.  Dried herbs like oregano, thyme, or bay leaf are good and celery seed adds a nice flavor to chicken.  If you want to bring something unexpected to the flavor try adding some ground coffee, cocoa powder, or both.  

Here are a few rub versions I have made along the way.  I would still recommend tweaking them to match your tastes, but it will cut some time off the evolution.

Remember to mix well breaking up any small lumps with your fingers.

2 T kosher salt
4 T natural brown sugar
2 T cumin
2 T chili powder
2 T black pepper
1 T cayenne pepper
¼ C paprika

Basic Beef Rub
6 T paprika
4 T kosher salt
4 T black pepper
½ t cayenne

Basic Brisket Rub
½ C paprika
5 T natural brown sugar
3 T garlic powder
3 T onion powder
2 T oregano

Cajun Rub
2 T kosher salt
2 T paprika
½ t garlic powder
½ t onion powder
½ t dried thyme
½ t  dried oregano
½ t black pepper
1 t white pepper
1 t cayenne pepper
½ t ground bay leaf

Yard Bird Rub
1 T celery salt
1 T paprika
1 T garlic power
½ t black pepper

Key: C=Cup, T=Tablespoon, t=teaspoon