Ideally you need two sorts of objects:
1) things which absorb sound waves: Curtains. Rugs. Large lamps. Chairs etc.
2) a reduction in the amount of purely horizontal and vertical hard surfaces. Adding things like framed pictures, which are hung a few degrees from vertical, will help to deflect sound waves instead of bouncing them from one hard surface to another.
The cheapest option: a white noise generator. You can either buy a dedicated one on Amazon for like $50 or you can just have a computer play a white noise mp3 in a loop.
Another low cost option: more furniture that's made of materials that will absorb sound. This would include sofas, curtains, rugs, etc.
Next best option: acoustic panels. If you can't afford to get enough for the space, you can check YouTube for videos on how to make your own.