Here's some suggestions, having been a regualr cyclist:
At first, if you haven't cycled much, you might consider regular loose cargo or board shorts. If you're going to do it long term, get some decent lycra shorts. These are not inexpensive. Name brand (Pearl Izumi) can go for $80 or more. $50-55 fror biking cargo or board type shorts.
Lycra shorts are very good because they stretch with you as you move, thus preventing friction and rubbing - very useful when you're biking from more than 30 minutes.
Lyscra cycling shorts also have a material that wicks moisture away from your body yo prevent more discomfort. Furthermore, some have energy absorbant cushioning in the seat of the pants (as well as the saddle) to help you with saddle soreness which in the beginning can be a real issue.