To display top scores in your game you can simply use get_top_n_rankers. In Which you can also short your game scorer by adding custom header for documentation get_top_n_rankers_by_sorting.
To get total scores of user you can use get_scores_by_user
If you want to get sum of user scores you can do this by using our storage service. In which you can simply insert your user score in JSON format and get the sum of score by using map_reduce function like this.
String result = storageService.mapReduce("<DB Name>","<Collection Name>","function map(){ emit(this.userName,this.score);}",
"function sum(userName, score){ return Array.sum(score)}}");