In order to retrieve all the content in your private folder, first thing you have todo is to retrieve a valid token via login.

Once you have obtained a valid token you have to invoke a findByProperties with the following request body:

{
	"client": {
		"clientId": "<your clientId here>"
	},
	"criteria": {
		"contentType": ["VIDEO", "AUDIO", "IMAGE", "OTHER", "PLAYLIST", "PAGELET", "URL"],
		"acl": {
			"onContext": "DIRECT",
			"rules": ["SEEN_BY"]
		},
		"linkedCategoryOp": {
			"linkedCategoryIds": ["!__TRASH_"]
		}
	},
	"orderBy": "lastUpdate_D",
	"contentFieldOption": {
		"returnLinkedContents": false,
		"returnLinkedCategories": false,
		"returnThumbnailUrl": true
	},
	"offset": 0,
	"numberOfresults": 50,
}

Please remember that the request is paginated, only the first 50 content will be returned; increase the offset to retrieve the subsequent content.