do your bit
do a/(one's) bit
1. To do a small amount of something. In this usage, "a" is used between "do" and "bit." I was only able to do a bit of cleaning before your mother called—that's why the dirty dishes are still on the table.
2. To contribute work to that of others in a group. In this usage, a noun or pronoun is used between "do" and "bit." If you don't do your bit with the slides, our presentation will never be finished on time.
do your bitmake a useful contribution to an effort or cause. informal
The exhortation to do your bit was much used during World War 1, but the expression was current in the late 19th century.