Boise has a respectable vacancy rate, so you should always be able to find an apartment. Most online listings show that they are listed for immediate availability. As long as you start looking a month before your move in date, you shouldn't have any problems finding a place to hang your hat.
Preparation is everything. If you show up for a meeting with a landlord and have a copy of a credit report, a resume, and reference contact information in hand, you're going to make a very strong first impression. This can get you in the door before the landlord even starts looking at competing applications.
If you don't have a car, location is going to be everything when you're searching for an apartment in Boise. Research where you're going to school or working and make sure you get a place that is within a reasonable walk, as the transit system can't be relied on to provide good reach throughout the city. If you're a cyclist, the Boise Green Belt provides you with a perfect cycling path that cuts across the entire town.
Downtown is where the action is in Boise. Although we've all heard jokes cracked about the bustling nightlife in the capital of Idaho, the reality is that the nightlife is pretty great compared to comparably sized cities. Whether its an Irish pub, live music or a serious club you're looking for, you can find it downtown. For outdoor enthusiasts, white water rafting in the summer and nearby skiing at Bogus Basin in the winter should satisfy your need for adrenaline.