When you're learning to play, start with the decks recommended in the Learn To Play guide.
Next, putting all cards from one faction + the neutral cards into a deck, and then improving it by slowly substituting cards from other factions to shore up weaknesses and improve synergies, is a good idea. (Now keeping in mind the actual deck building rules and influence.)
If you want more pre-built decks, check out netrunnerdb.com and search for decks that contain just the Revised Core.
Once you feel like you have exhausted the possibilities of the core set, buying a big box expansion can be a good idea. If there are specific factions you like, buy the corresponding big box:
- Creation and Control: Shaper and HB
- Order and Chaos: Weyland and Anarch
- Honour and Profit: Jinteki and Criminal
- Data and Destiny: NBN and Runner Mini Factions (This one is NOT good to start with because the Mini Factions need a larger cardpool to be good!)
Terminal Directive has cards for 4 out of the 7 factions and contains a campaign to play with another player (only do this once you are comfortable with the normal game!). The campaign is fine, not great, but the tournament-legal cards in that box are good for padding out your cardpool.
If you really want to start with data packs and cards for all factions, check out the "Shipment From ChiLo" Buying Guide, here: ChiLo Buying Guide