You can obtain Sentinel 2 data from the ESA and from the USGS. Users must register for free access to data at either site. Currently ESA distributes Sentinel 2 data in entire swaths. These files are very large, sometimes 7+ GB when compressed. You can access these data directly from the ESA Copernicus Open Access hub at: https://scihub.copernicus.eu/ Users should read the online User Guide prior to searching for data.
The USGS distributes most of the Sentinel 2 data but has cut these data into much smaller tiles of 100 km X 100 km in the Level-1C top of atmosphere reflectance. These data are much easier to work with and users are encouraged to look for data here first. You can access these data at the USGS Earth Explorer site at: https://earthexplorer.usgs.gov/
You should begin your data search by first defining a small region of interest. You can expand this later as needed. You can enter a start and stop date to further limit your search. Next in the Data Sets tab scroll down and select Sentinel | Sentinel 2.
Click on the Results tab to display data that meets your search criteria. You can click on the Footprint icon to show the scene coverage. To the right of this is a Browse icon. This will display the image to help you decide if the scene meets your needs. Users are encouraged to only download one or two images initially. Once you gain experience working with these data you can come back and get more.