When you are using online wallets either on your mobile phone or personal computer, you need to make sure that you don't download or install untrustworthy applications or anything, don't click unknown links, don't give access to your device to any app even if it's for a short period because these things can compromise the security of your device and funds.
If your wallet is installed in the same device you use to browse the internet, then it is prone to hacking and you cannot be too careful. You could download a phishing application or click on a link that has malware and lose your funds to scammers. Instead of trying to be too careful, store your funds in cold storage if it is worth so much, and use online wallets only for money you need for spending on the go.