Can I use Electrum on my everyday computer? Or do I have to use a computer that has never used the internet?
There's nothing stopping you from using Electrum on an online computer. If you have a lot of coins and want to have better security, you should use an air gapped setup where the offline computer has the private keys and has never been online nor will ever be online. Or you could use a hardware wallet. But using Electrum on an online computer and using a strong password (on your computer and on your Electrum wallet) is also fine.