To air seal a typical home, you could caulk and weather seal all the gaps and cracks in the thermal envelope (electrical outlets, around recessed lights, attic access, doors and windows, and around HVAC boots). The big holes will most likely be the band board (where the floor joists hit the outside walls). These are best sealed with spray foam insulation (open cell not closed cell).
Then, most likely your attic does not have enough insulation and probably not air tight insulation. This is a hard decision to make – to fix the air tight issue you usually need to remove the insulation that is already there. Most people, including myself, don’t like throwing away something you already paid for. So typically, you just accept the air leakage in the attic insulation and add another layer of insulation on top of what you have there. Code minimum is R-38, but I would suggest a minimum of R-50, but like to see R-72 if budget and space allow.
A basement / crawl space is another good place to attack if the space is not finished. We often find insulation in the floor system above. It should be on the walls for better performance. The band board is often not insulated at all, this area should be sprayed foam.