So you want to hack on Istio? Yay! Please refer to Istio's overall contribution guidelines to find out how you can help.
To make sure you're ready to build Envoy, clone and follow the upstream Envoy build instructions. Be sure to copy clang.bazelrc into this directory after running the setup_clang script. Confirm that your environment is ready to go by running bazel build --config=clang --define=wasm=disabled :envoy (in this directory).