In order to become an S corporation, the corporation must submit Form 2553, Election by a Small Business Corporation signed by all the shareholders.
Generally speaking, if it's later in the year, wait to form your S Corp until the following year, unless you expect a significant spike in income. If it's earlier in the year, talk to your tax preparer about how much you could save by switching mid-year and see if it's worth it.
LLC taxed as an S corporation First, an LLC would need to elect to be taxed as a corporation by filing Form 8832, Entity Classification Election. After that, an LLC can then file a Form 2553, Election by a Small Business Corporation, to elect tax treatment as an S corporation.