Can I send memecoins straight to an exchange?

Usually not. Exchanges generally support well-known tokens, so swap to SOL or a stablecoin first.

Do I owe tax when I cash out?

It may be taxable depending on where you live. Wallets do not issue tax forms.

Swap to a widely supported token. Exchanges generally support SOL, ETH and stablecoins such as USDC or USDT, not lesser-known memecoins. Phantom's help center advises swapping those first.
Get a deposit address on the exchange. Copy the address and the network, and any memo it needs.
Send from your wallet. The token and network must both be supported, or funds can be lost. Keep enough SOL for the network fee.
Sell and withdraw. Sell for your local currency on the exchange and withdraw to your linked bank account.

Phantom-specific steps and the US Cash route are in how to withdraw from Phantom.

Blockchain transactions cannot be reversed.
Selling for cash may be taxable depending on where you live. See the tax tools page.
Closing empty token accounts returns their rent deposit, see what Solana rent is.