Hey Christina,
You're probably looking for a Lead Performance report, grouping by Lead Created, and then with custom columns for score ranges.
The first step is going to be creating a few smart lists, up to five, with a Lead Score Between X and Y. Between is inclusive in Marketo so if you set Lead Score Between 1 and 50, your next list should be lead Score Between 51 and 100.
You'll then want to create a Lead Performance Report, going to Setup, setting your Created Date, and grouping by Created At Week. Then you'll need Drag in the Custom Columns, and select your Lead Score Smart Lists.
This will create a report for you showing the total volume of leads created during each week with the number of leads in each Smart List based on their lead score range.