Brass, an alloy of zinc and copper (and sometimes other metals) has been used worldwide for thousands of years to create decorative items. Value comes with age, demand of a specific kind of piece, rarity, and quality. Brass objects made by known artists will be signed or marked and can be quite valuable. Mass produced figurines that look clunky or have ill defined features are low quality and hold little value. There are many such items for sale online so you can check them out for comparison.
Check out books for more information:
"Antique Brass and Copper Identification and Value Guide" by Mary Frank Gaston
"American Copper and Brass" by Henry J. Kauffman
"An Introduction to Brass" by Eric Turner
That is just to name a few. There are many more. Do not rely on old price guides for reliable information as values change over time.
Whether you decide to polish off the oxidation or not is up to you. Use Brasso and follow instructions on the container. Use a soft cloth. Do not use abrasive materials to clean the brass. You may want to identify your pieces before you clean. Some collectors prefer bright, shiny items while others prefer the vintage look of oxidized brass.