How do I mirror what I am doing on one part/side of the mesh onto the other? I am trying to make a symmetrical mesh were everything I do on one side get mirrored onto the opposite side (opposite axis) side

I came across many tutorials explaining how to mirror what's on one mesh onto another but these are from pre 2.8 and what I neeed is for the same mesh to be mirrored just on different sides.
Basically you need to add Mirror modifier to your object. As mirror center you can use object origin point on any other object by select it in Mirror Object. 



You can adjust object origin using Object > Tranform > Affect only > Origins