•What damages can a malware cause to your wallet?
Well it will drain it, of course.
•What are the signs that your wallet being attacked by malwares?
Difficult question, as it is very difficult to detect a malware infection as they usually just download the wallet file to their own servers and do the cracking there.
•How can it be prevented?
Use an antivirus and don't click on shady links such as "GET FREE BITCOINS HERE" or "AIRDROP" and be careful with pirated software. And always update your software to the latest version as well as using a hardware wallet to store large amounts of funds.