For small amounts its fine to use the various exchanges and web wallets but for large amounts that you want to hold long term it is absolutely crucial that you buy a good hardware wallet such as Trezor. Once you own your keys no one can steal your coins. Even if you someone has your Trezor and keys they can not get your coins if you also enabled an encrypted passphrase. Stay safe, invest in a wallet, after all if you are investing thousands then $100 for a hardware wallet is well worth the piece of mind.