The most secure way to hold all of your altcoins, is via a hardware wallet. This is offline storage, so it's only in your hands. The top two choices are without a doubt Trezor (click the above banner) and Ledger, here.
Buying and selling your altcoins, Bitcoin especially, can be a pretty lengthy process.
Simply, the best and easiest ways are either via a site such as Coinbase, where you go through identity verification and set up your payments options, LocalBitcoins or Paxful, where you deal directly with other people via PayPal, Google Pay, etc or CashApp. The process on CashApp can be lengthy, so if you need Bitcoin now, Coinbase, where you go through identity verification and set up your payments options, Paxful or LocalBitcoins will be the best option.
There's also the option of posting a currency exchange, here on ForumKorner!
Looking for more information on how to buy, sell or trade crypto? Please visit the exchange page.