You get it from either setting up your own custodial wallet, which takes some effort and know-how, or you can use a custodial wallet with all the fun features built into it so you can start right away.

Here's a bit more info: https://nostr.how/en/guides/setup-zapping-wallet

Others have mentioned Wallet of Satoshi if you're not in the US. You can also try an #ecash wallet that supports whatever device you post on: https://docs.cashu.space/wallets . Ecash tokens are redeemable for bitcoin.