Imagine a fantasy world, filled with elves and dwarves and the walking undead. Now imagine that world heard about American Football, and decided to form their own idea of the game. There are teams, and kick-offs, and touchdowns, but there's also violence and death. The goblin teams like to sneak weapons onto the pitch when the ref isn't looking, and the vampires have a habit of snacking on their own teammates. Now scale that down and make it a strategic tabletop game, and that's Blood Bowl!
Blood Bowl was created by Games Workshop and exists off to the side of their Warhammer fantasy universe. So you can have a team dedicated to violence and bloodshed in service of the Chaos God Khorne, or a high-speed, easily breakable Skaven team. But you don't need a deep knowledge of the Warhammer lore to be able to play, or a big army of miniatures.
A game of Blood Bowl needs a few things: two teams of miniatures (one for you and one for your opponent), a pitch to play on, a number of dice (including some specialised block dice) and a copy of the rules. But how much of that you need to get started depends on how much you're willing to spend and how committed you are to the game.
If you're certain that you want to play and don't mind spending money, then the most complete way to get started is by purchasing the box set (currently "Blood Bowl Third Season Edition"). That comes with everything you could possibly need, including the rules, the pitch, two teams of miniatures and two sets of dice. It's a great starting point that ensures you won't need to buy anything else in future except maybe more teams once you have a feel for the game and want to try something new.
If that's too expensive, or you don't want two sets of miniatures for your own use, things can be bought separately. There are also third-party retailers who offer alternative team models, either pre-made or as files for 3D printing, and dice. These can be cheaper options, especially if you're only buying what you absolutely need to play.
Page contents wilfully stolen from the excellent Orkshire Blood Bowl League site