How to pay for Dex?

Hello, I'm curious about how to pay for transactions on a decentralized exchange, or Dex, as I've heard they're gaining popularity in the crypto world. Can you walk me through the process? Are there specific cryptocurrencies I need to have in my wallet to use a Dex? And how do I ensure the transaction fees are reasonable and won't eat up too much of my investment? Additionally, are there any security measures I should take when using a Dex to protect my funds? Thank you in advance for your help.