How do I buy from Dex?

I'm interested in purchasing assets through a decentralized exchange, but I'm not quite sure how to go about it. Can you please explain the steps I need to take to buy from a Dex? Is there a specific platform you recommend? What kind of security measures should I take into account when using a Dex? And finally, are there any fees associated with buying through a Dex that I should be aware of?