If you live in the USA Gemini and Coinbase are your only decent options to buy crypto with USD. Use Gemini because the fees are lower. Don't use Coinbase with your credit card. Use Coinbase Pro and deposit money with your bank account (ACH) and wait for it to clear. Then place a buy limit order to minimize fees.
In crypto security is your own responsibility. You absolutely must use 2FA on any exchange you have money on. Write down the code with pen and paper. Read from the paper and put the code in your phone's Google Authenticator. Keep the paper backup in a secure place like a safe in your house or a bank deposit box. If you haven't done this in step 1 do it now.
It's good to use a VPN for added security and to hide your location.
These are three of the best exchanges. They have many markets, low fees and margin trading