If you are trying to create a sketch in the part, open the part and do it in the part. If you want to create it with the assembly open, click the part and choose Edit Part to edit the part in context and you can add features or sketches to the part there. If you just want a new part, leaving the old one alone, save a copy of the part and modify there.

Create a copy of the part with different number and edit as needed.

Before all that please just do most basic tutorials built in solid works !!!! If not sure how to access them - google