Hello! Google has developed a photo uploader, which I highly recommend in order to automate the upload process as much as possible,
First, go to the Google Photos website and download the Desktop uploader. Install the app and then log into your Google account.
You'll then be prompted to "choose backup sources". Select the folders here that you'd like Google to "monitor" for changes; any photos in -- or later added to -- these folders will be backed up to Google Photos automatically.
I recommend leaving the "Photo size" set to "High quality (free unlimited storage)." Click OK and the photos and videos in the selected backup sources will start uploading into Google Photos.
If you'd prefer to manage your photo uploads manually, you can do so by going to photos.google.com, logging in and clicking the little cloud button with the "up" arrow inside of it, which is located at the top of the screen.
Alternatively, you can drag and drop files or folders directly into Google Photos to upload them