It depends on what you mean by a "big year."
If you're just aiming to see as many species as you can in a given year, then it's not necessarily "hard" at all. Many/most birders are doing a big year every year.
If you're actually trying to get the record or a really high number for a given area, then you'd be hard-pressed to succeed without a vehicle. Much of the strategy involved in going for a big year is quickly getting to rarities that other people have found, so a vehicle is pretty important. Add to that the fact that you're a new birder, and a high-number big year is probably a lofty goal.