What is the size of your case? If it is too small, upgrade to at least a mid-tower with a place for a 120mm exhaust fan in the back. Use the largest fan that will fit the case holes, moves a lot of air (higher cfm), and is not too loud (lower db).

If you have good case air flow, then you need to address your heatsink and fan on the CPU. Replace the heatsink with a highly rated model that can hold a large fan (90-120mm). Remove the old thermal paste and apply Arctic Silver 5 as per its instructions.