If you have a wallet, click connect on the left to connect your Metamask wallet to buy Aerdrop. If you need BNB, then click on the 'Buy BNB' button.
Once you have BNB, select the amount you want to spend or click Max (remember to save at least 0.01 BNB for GAS later). You'll see then that the amount of AER to buy is populated and you can click 'swap' and there will be a popup in your MetaMask to accept.
Click add token to Metamask.
If you don't yet have a wallet, keep scrolling.
MetaMask and create a wallet. Keep your phrase a secret! Never share it with anyone and store it properly!
Go to Chainlist, connect your wallet and in the 'Search Network' box enter the 'binance smart chain'.
Two options will appear, under the one with ChainID 56, click 'Add to Metamask' for this to appear in your wallet.