It depends on the airdrop event requirements, basically all you have to do is to register and fill up given form with your details like name, email add and wallet address for the reward and then thats it, you just wait for the token reward but I observed recently that airdrops now usually requires telegram accounts. Aidrops are usually time constrained and has a minimum number of participants so you its normal if most of the airdrop event you see are done before you can participate. Take note, there are also some reports of airdrops being a way to get your account details so make sure you fill up forms that do not require you private keys or to download links.