First:
Sour mix and Margarita mix are not the same. That being said, you'll produce a much better drink if you learn how to mix a drink without that colored sugar water.
Classic Margarita drink:
http://www.seriouseats.com/recipes/2015/04/classic-margarita-recipe-tequila-cocktail.html
On building a bar:
My advice is to just pick a few classic drinks that you know you and/or your close friends will enjoy and buy what you need to make those drinks. Try to pick 1-2 classic drinks that use one (or more) of the following base spirits:
- Gin
- Tequila
- Rum (white or dark)
- Vodka
- Whiskey (All bourbon is whiskey, but not all whiskey is bourbon)
- Scotch (for serving neat, or mixing. Yes, you can mix scotch without the world ending. Google blood and sand for a great way to enjoy a smoky scotch.)
If you're not sure what brand to get, go to your favorite bar at a slow time of day, order a beer, and ask what spirit brands are crowd pleasers. If you're in good standing with the bartender, they may give you a few small sips to sample for free. Make sure you leave them a very nice tip for their time.
Don't buy anything you wouldn't drink (unless for a very close friend that comes over often) because then you're just wasting space and money. (Most) Liquors will last on the shelve, but liqueurs and mixers will go bad if left at room temp after opening. If you're not sure if it needs to be refrigerated after opening, put it in just to be safe (sweet and dry vermouth should be kept cool after you open it!). Buy only what you think you can use in three months; it's cheaper to keep buying the smaller bottle every few months than it is to pour out a giant bottle that spoiled. Same goes for bitters, don't buy every bottle you see; just get classic bitters and wait until you learn how to make other drinks that call for more exotic ingredients.
Don't go crazy trying to buy everything all at once, buy what you need to learn a new drink every weekend or so until you know 5-7 drinks off the top of your head. Your cabinet will be stocked with plenty of options at that point and you'll be able to make your standbys without having to stop and look up the recipe.
A drink is only as good as the sum of its parts. Use good quality spirits and if a drink calls for juice try your best to
make it fresh (lemon/lime is stupid easy to squeeze). Simple syrup is easy to whip up in small batches, learn how to make it (hint: its name should give you an idea of how hard this is).
Don't deviate from the recipe until you've made it a couple times, then start to experiment.
Cheers.