Tap water in Zambia is generally not drinkable, at least unless boiled. Bottled water is widely available in cities, but not necessarily in rural areas.
beer
Zambia's best-known brew is Mosi, a clear 4% lager available everywhere. Eagle has more taste and more kick at 5.5%, while Zambezi Lager is a microbrew worth sampling if you run into it. The South African brand Castle is also bottled locally, and all of the above run around $1 in a store or $1-2 in a bar.
If you are near the borders, you are likely to find Carlsberg good, from Malawi, Simba excellent, from the Democratic Republic of the Congo, Kilimanjaro nice lager, from Tanzania, and Tusker strong, from Kenya. Other imports can be found in larger markets but will also cost more.
soft drinks
A traditional local drink worth trying is maheu, a somewhat gritty and vaguely yogurty but refreshing beverage made from maize meal. Factory-produced maheu is sweet, comes in plastic bottles and is available in a variety of flavors including banana, chocolate and orange, while homemade versions are usually unflavored and less sweet.
Coke products are accessible and cheap at less than a quarter a bottle, but beware of the deposit system: in rural areas, you may have to return an empty bottle before they'll sell you a new one!
local alcohol
The locals' drink of choice is masese muh-SE-say or ucwala uch-WALA, also known as Chibuku after the biggest brand, made from maize, millet, or cassava and resembling sour porridge in texture and taste. If you want to try this, it's best to look out for the factory-made kind in milk-carton-like containers.
In rural areas, there are opportunities to drink local "homebrews." A wide variety of homebrews exist in Zambia, from beers made from honey in the Southern province of the country, to wine made from tea leaves in the Eastern portion of the country.
Finally, there is kachasu cuh-CHA-suh a spirit distilled from anything Zambians can get their hands on — including battery acid and fertilizer. For obvious reasons, therefore, it is better to avoid this moonshine.
On a final note, most men at bars are relaxing, while many women at bars are working. Therefore, if you are a single woman in a Zambian bar, be aware that you might be approached and offered the opportunity to do something you did not intend to do.