The forms that apply to your entity
1120, 1120S, 1065, or a Schedule C: which one you file follows directly from the entity choice made at formation.
Federal deadlines that do not move
Quarterly estimates, the extension that buys time to file but never to pay, and what happens when both are missed.
State and local obligations
Nexus, sales tax, and franchise tax in every state the business actually operates in, not just the one it was formed in.
Planning before December 31
Retirement contributions, depreciation elections, and the moves that stop being available the moment the tax year closes.
Written for general guidance. Thresholds and deadlines differ by jurisdiction and change often, so check your own position with an advisor before acting on it.