Help soundproofing already built room
I've attempted to build a reasonably soundproof music room in my basement and can tell you it is a tall order.  I built a room when finishing my basement using double drywall with Green Glue, a floating double drywall ceiling with hat channel and isolation clips, sealed outlets, and re-routed HVAC and it I can still only play at a moderate volume without disturbing the peace upstairs.

The best thing you can do is build a room within your room that is structurally separated from the existing walls and ceiling.  Fill the space in between with Roxul Safe and Sound Insulation (Lowes). It's the best option but still has limitations and is not cheap.  There are many examples if you Google it.

If the room already exists and you're on a tight budget, you can just do your best to hang heavy material on the walls like quilts, etc. and isolate anything that generates sound from the surfaces of the room with rubber mats, mouse pads, etc. Also avoid cutting holes in the ceiling for recessed lighting etc. Use lamps or surface mount lights. Anything you can do to reduce vibration (sound) from getting to the existing structure of your house will help.

Hope this helps.