This is what I generally use - I wouldn't say all of it is absolutely neccesary tho.
- A few philips head screw drivers (magnetised ones are ideal) -for most of the work, usually one of the right size will do the job
- A big, solid screwdriver (possibly with swappable heads) - for installing fans and spring mounted heatsink screws, these usually require a lot more torque than the rest of the build
- Needlenose pliers - great for insalling fiddly connections or picking up screws
- A desk light - often ceiling lights and natural light is enough, but it is usually good to have some extra illumination over the case when building.
- A tube of thermal paste - for installing between the CPU and heatsink to provide good thermal contact, I use
this stuff but many other options are available
-
Thermal paste remover - makes life so much easier if you need to re-apply thermal paste.
-
can of air - great for knocking out dust when working on a PC that is not brand-new
- microfiber cloth - for cleaning CPU and heatsink surfaces without leaving any fluff
- big sheet of cardboard - so you can assemble the core system on this first to ensure it all works, then rest the case on cardboard so neither your case or table gets scratched.
- A nice meal beforehand - nothing worse than getting hungry during the middle of a build, you will most likely want to do the thing in one go.