If you are referring to a rain garden, Extension some introductory resources (see link), but you may want to hire a professional to provide on-site consultation or do the actual construction of the garden, since those that aren't built correctly can fail and retain water when they should be draining after a given period of time. The size (surface area plus depth) of the rain garden to build will depend on the amount of water you want it to be able to capture and filter.

One option for candidate hires may be a Chesapeake Bay Landscape Professional, as they have training in sustainable landscaping techniques (including stormwater management), though we grant you that its focus is closer to the Bay than you are located, so we don't know how many CBLP-certified persons work in Washington County. You could also reach out to the Washington County Division of Environmental Management watershed specialist, Alex Reed, for sources of additional local resources.

Miri