The default decks are the decks you get by shuffling together all the cards for one faction with all the neutral cards for its side (Runner or Corp). They are generally agreed to be fairly well balanced against each other, so they’re basically fine if your opponent is also playing one of the default decks. When you start playing opponents with constructed decks, you’ll find the default decks somewhat lacking.
There are lots of single-core-only decks on NetrunnerDB, and they can easily be found by the search, but unfortunately many of them are meant for teaching/demo games rather than regular play. Also, they generally are not MWL-compliant (and therefore are not tournament legal).
The beginner decks I’ve linked to are not teaching/demo decks, but many of them are not MWL-compliant.
The NBN decks are actually no longer legal even for casual play, because the latest errata limits Astroscript Pilot Program to one copy per deck. Casual play opponents are pretty unlikely to object to this, since you don’t have the cards to work around that errata, but you should warn them when you’re playing a slightly illegal deck.