You must declare a major by the end of the sophomore year. Get a "declaration of major" form from the Registrar's Office.
You can't declare a minor until after declaring a major.
It's possible to drop a major (or minor) and add another. Doing this too late in your academic career may require taking extra courses to catch up!
If you're interested in a Self-Designed Major, see Dean Breitborde early on to discuss your ideas and the approval procedures. Dean Breitborde chairs the Curriculum Committee which will eventually review your proposal.
If you're interested in a Self-Designed Minor, get a Self-Designed Minor Form from the Registrar's Office to complete. No committee approval is required.
Some academic majors have more "linear" or "sequenced" requirements than others. That is, you need course 100 before you can take 101; you need 101 before you can take 201, etc. If you think you have an interest in a particular major, take a look at course prerequisites (they're listed in the Catalog) to see how flexible the requirements are. That knowledge can help guide your choices term-by-term and determine how much flexibility you have to explore other fields.