Hi.. heres a few pointers for you ..
The first thing you need to do is decide what games you are going to use then make sure the pc parts are good enough , you can do it like this , for example you mention fallout 4 so type into your browser ... what do i need to run fallout 4 , you will get 2 different types of answer , some websites will just list the components you need to be able to run it and some sites have little programs for you to run , they will scan your pc and it will say if you can run it .
Depending on how old the game are you might have to run the game in compatibility mode , their are 2 ways to do this , to do this you might have a short cut or a program launcher , in both cases right click on them and go through the tabs and look for the one that says compatibility mode. once you open that click on the pull down tab and you will see a list of previous operating systems , choose the one that was in use when the game was made .... you have now technically fooled the game into thinking it is running on an older machine and not a brand new one
99% of the time this will work but occasionally it wont because your brand new pc will be too good for the old game. Example ... you could for example jump from a cliff onto another piece of ground BUT instead of landing on the other piece of ground you will go through it and the game will crash .... just something to be aware of.
I wont give you the technical reasons why this happens because you said your new to pc's