After you sign-in to your account only preference option is enabled. After signing the sync process will start to download all files. You can pause that action and click preference it will show which folder you need to sync under the "Sync options" tab.
You should select the "Sync only these folders(n MB selected)" and below this you have to check only the particular folder or sub folder to sync to Google drive. Other folders will be deleted from you local if you have downloaded already but will stay in your Google drive.
Any modification made to this sync folder under Google Drive directory will also be updated and synced automatically. Hope this helps.