Supply USDC to Aave contract in order to get aUSDC

You can supply USDC into to Aave and receive the equivalent aUSDC by calling the supply() function in the Pool contract. You need to already have USDC in your wallet. The documentation shows you will need e.g. lendingPoolContract.supply(usdcAddress, usdcAmount, addressToReceiveATokens, referralCode)

To call supply(), the Aave pool contract should be able to move your tokens on your behalf, otherwise the contract can't supply your tokens. So you must approve() (or permit for Aave v3) the contract to supply your tokens first. Approve() is a function from any ERC20 token so you can use that.