how to used same sketch (share) on different part

A decision needs to be made. Should the sketches all update when/if a change is made in the future to an original sketch? Or shall each sketch simply be an independent copy with no link or association back to the an original?

I'd usually advise against making the sketches interrelated unless someone knows exactly what they are doing. Making a change to a model, and having it change parts that were designed months ago can be a terrible situation.

When I want to reuse a sketch, but don't need it to update, I simply open a part with the sketch I want, copy it, then paste it into my new part.
Ideally, I'd set this up as a library feature in SolidWorks, but I don't need this functionality often enough to bother with a library of sketches.

Related sketches can be done in a number of ways, but the method to use really depends on what you are making, and the reason to link the sketches.
You might:
- use configurations in a part file. This method could make use of the same sketch, and cause a change to ripple through all the other configurations.
- Use a master sketch in an assembly. Here the assembly holds the layout sketch. You can then create new parts in the context of the assembly, and have those new parts be related to the master sketch. Then, any change to the master sketch will ripple through all the associated parts.
- Derived Sketches are cool, but that is mainly limited to reusing a sketch within the same part.
- Derived part could be used. In the master part you'd places your sketches. Then you'd derive parts from it. In this case all of the derived parts are linked back to the original.

You can break or lock these interrelationships between parts in the future, but unless you (and everyone else with file access) knows exactly what they are doing, derived, or otherwise linked parts often lead to a disaster, or at least unintended consequences.
Like I tell my students "It is a powerful tool, like a gun. It is a very good tool to have, but you could very easily kill yourself with it if you don't know how to use it"

Well, if you can explain the process for both option I will more than happy!

Copying the sketch is really useful. But when you think of reusing a sketch for me, the best way is to use "sketch blocks". I save a library of sketch blocks or I save the blocks in the assembly file.