Electrum already creates a wallet for you that has a seed phrase that you can write down and use to recover your funds in the future.
it is best to write the seed down but if you wanted a single key you can still right click any address and get its private key (in your Address tab).
Just keep in mind that Electrum seed works only in Electrum.
If you want to create a seed that works everywhere, I'd suggest Ian Coleman's tool (
https://github.com/iancoleman/bip39 )