I run bitcoin core. What should I prepare by August 1st? I'm not sure about UASF/BIP-148 but should I do something for the activation? Will bitcoin core dev release new version? I do support segwit by the way.

My bitcoind servers are used as a provider of utxo info and accepting signed transaction to broadcast. The wallet and mining feature are not used.
According to UASF Working Group

What do users need to do to enforce BIP148?
It is recommended that users do not update unless an economic majority commits to updating and users are aware of the risks and mitigations of a failed UASF deployment.
Users aware of the risks and who want to commit should use clients that enforce BIP148. Users that run full nodes would upgrade to one that enforces BIP148, or run their node behind an upgraded border node. Users of light clients (like mobile wallets) should check with each vendor to see their support for BIP148. We plan on documenting any public responses from wallets regarding BIP148 support.

This web-page describes the mechanism for any User Activated Soft Fork (UASF)