IMO the best way to go for starting rebuildables is one of the inexpensive Chinese atomizers.
The easiest to rebuild are the RDAs - rebuildable dripping atomizers. As the name implies, there is no tank, you drip juice into them, vape, repeat. The Igo-L or Phoenix are good atomizers to, at least, start out with.
For a genesis-style tank atomizer, the AGA-T or a DiD 'clone' such as
this one.