Un-box your bike and quickly check for obvious damage or missing parts. Some bikes are put together better than others but here’s a few quick things to remember.
The bike should be safe. Critical parts must be tight. Putting grease on threads will generally help tighten parts down as well as keep them from siezing in the future. Seat posts, Stems, Pedals and Bottom Brackets are rarely greased/ lubed. Use as much grease/anti sieze/ carbon prep as possible. Be sure to do this to ensure the continued functioning of the bike well into the future. Nuts/ Bolts should also be greased if they were not at the “factory”.
If you have many of the same model bike you may quickly find out what was and wasn’t done durring factory assembly which can save some time. BB threads are notoriously under lubed and tightened (see BB section for more specifics). Using longer handled tools, foot, vise, torque wrench can help.
Develop a routine. Working through the bike the same way will help ensure that things are tight and safe as will tightening down anything you install even if it is mispositioned such as a stem. You should develop a mental check list (as well as a physical one initially) to ensure safety and consistency with your builds. Typically working back to front as more issues often can arise there- more important perhaps on used bikes, but this helps with consistency (The rear wheel, brake, derailleur etc. being a bit more critical/ likely to have issues than a front wheel, headset etc).