To get started with imports, we recommend that you review this article: Import Records. We include screenshots in our steps so you know exactly where to click while going through the import process.
You’ll want to be sure to read through the section on preparing your spreadsheet to ensure your import goes smoothly. From there, you'll see steps for either creating a new object with data or for importing data into an existing object.
If you're running into an issue with your import, then check out our Imports FAQ section for overcoming common import issues. That section also covers common import questions you might run into.