How would I make a force field that can block entities from passing through?  I don't even know where to start

if u dont know how to start it might be 2 hard 4 u.

a force field is nothing else then a lot of blocks that are unpassable.

Well I want it to surround and follow the player. It would be too inefficient for blocks to do that

hmm... maybe use playertickevent get all Entites around the player and push them away from the player

How would I make a graphic for it?

thats beyond my abilities. u need to deal with open gl, which i have never done. maybe ull find sth on the internet

Depending how complex you want, you can just watch the playerrender event and draw a transparent box around the player.  Do a search, people have done this before.