Buy and set up a hardware wallet like the Ledger Nano S. If it is too expensive for you or is on backorder, the next best option is to create a paper wallet. Transfer the majority of your lumens to your hardware or paper wallet. Note: always transfer ~2 lumens first to make sure you are performing a transaction correctly, and store your hardware/paper wallet backups in multiple locations.

Since an offline computer has no connection, it is extremely hard for someone without physical access to it to access the account’s keys. However, this also makes every transaction an extremely manual process. - Stellar Developer Docs