If you are only able to by one and not the other, go for the dry one. For the most part, you'll need lemon, lime, and orange juice. Other recipes call for cranberry, tomato and grapefruit juice. Having plenty of cola, tonic water, and other sodas is definitely a good idea. Many drinks call for one of these, and it's also nice to have something to offer for the non-drinkers.
Most bars fill up glasses with ice before adding anything else.
Also, always used cracked ice, never cubes. Not only do pouring caps look more professional, and make it appear as though you are pouring a generous serving, they are also a lot faster to use than hassling with bottle caps. Most mixed drinks that don't have a carbonated liquid as part of the ingredients are shaken. Many open shakers are designed fit perfectly over the pint-sized beer glasses normally used for serving most draught beers. Strainers are paddle-shaped with slots for the liquid to drain through.