I want to make Ranking

Nice to meet you, my name is Beginner.
I’m Japanese, so my English may be wrong, but please forgive me.
I still can’t master Database and Repeating Group,
Please let me ask you a question because there is something I do not understand.

What I want to do
step1. Narrow down the database (number type data) to one month (narrow down the period)
step2. Sum up the numbers from the database narrowed down in step 1 for each user
Show step3.step2 in Repeating Group

I didn’t understand it as a beginner. Please tell me. please.