Ejuice is very simple as far as ingredients go. PG, VG, nicotine, and flavor. Since you make it yourself, everything is customizable, from PG/VG ratio, nicotine concentration, and flavor strength.
I get my PG and VG from Essential Depot on Amazon, but every DIY vendor carries it as well. For nicotine, go with Wizard Labs, Nude Nicotine, or Bull City Vapor. These vendors also carry many flavors, especially Bull City. They also carry all the mixing supplies you will need, such as empty bottles and syringes.
As roosterado said, check out the DIY section here on ECF. The folks in there are friendly and happy to help with any questions or problems you may have.
DIY E-Liquid