Welcome to the forums!
I'm glad you have managed to rescue this budgie. By looking at her, I can tell you that she's an adult female.
It's good that you are trying to find the owner, besides using social medial to advertise the found budgie, you can also place fliers around your neighbourhood and nearby vets and pet stores. It would also be a good idea to contact the local animal shelter.
It would be good if you could purchase a cage for her a smaller travel/hospital cage, these are inexpensive and would better accommodate her for the time being. She needs perches, containers to place food/water and a couple of toys. This is the bare minimum for a temporary situation.
You can start by checking this section: Housing for Budgies - Talk Budgies Forums
Essentials to a Great Cage
Since it seems she has poop stuck on her feathers, you can place a shallow dish of water on the tank to give her the opportunity to bathe if she wishes to do so.
When it comes to general care, you will find all the detailed information on the several articles and sticky threads located at the top of each section of the forums.
Best of luck with everything!