First, you’ll need to gather all the parts. The centerpiece of this project is the Raspberry Pi itself. It’s recommended that you spring for the 3 B+ or newer. You might also consider getting an enclosure to protect your Raspberry Pi from environmental damage. You can get a case online for fairly cheap, or those with 3D Printers can make their own. You’ll also need a power cable, an 8GB microSD card for storage, and some speakers to connect.
Speaking of connections, you’ll also need cables to connect the Raspberry Pi to the speakers. This can be tricky, though. The Raspberry Pi only has a 3.5mm audio output. If your speakers have a 3.5mm input, then great! If not, you can buy a 3.5mm-to-RCA cable or a 3.5mm-to-speaker wire cable in order to make that connection work. These cables are all fairly affordable and will certainly get the job done.
That said, you might consider using an audio receiver or an amplifier DAC as an intermediary between your Raspberry Pi and your speakers if you really want to take this setup to the next level. There is even one made by Raspberry Pi that bypasses the built-in 3.5mm jack and connects directly to the board. Buying one will raise the price of your project a bit, but a decent DAC will improve sound quality and potentially allow you to connect multiple speakers to a single Raspberry Pi for a much fuller sound.