It would be helpful if you could put more detail into your post. However, I will do my best to help you based on what info you have provided already.
A good thing to do is to open it up and replace the thermal paste on the CPU and GPU. Use brushes and compressed air to clean out all of the fans, air vents, and cooling pipes. Make sure all heatsinks and fans are attached and plugged in well. Reseat the RAM.
Use an application like cCleaner to clean out your storage, and defragment your hard drive. Close any nonessential programs and restart the computer.
Hope this helps!