What features? What do you have now (game consoles, blu-ray, roku / amazon / apple TV...) and are they all connected to the TV?
Some choices:
- Sound bar connected to TV (some include subwoofer, and some allow adding more speakers later)
- Thrift store used receiver and speaker set. More risky, but if you find good gear and it doesn't have any hidden problems this gets you the best sound.
- A cheap and not so good "home theater in a box" setup. Refurbished might get you a less-bad one.
- A 5.1 PC speaker setup connected to the TV's optical out like you'd do with the soundbar
The soundbar is the simplest choice and doesn't require any changes if your devices are all plugged into the TV now.
For the receiver and HTIB choices make sure they have enough HDMI inputs. Older receivers might also have only older versions of HDMI.