Snowbridge has initially only activated support for the sending of ERC20 tokens to Polkadot. To send Ether to Polkadot you can use WETH (Wrapped ETH). To register your own ERC20 tokens see Registering Tokens.
If you already have WETH you can skip this step. If not go to the WETH contract and make a deposit.
Weth Contract Address: 0xfFf9976782d46CC05630D1f6eBAb18b2324d6B14
Snowbridge needs to be an approved spender for you to send tokens to Polkadot.