How do I pay Uniswap?

Could you please elaborate on the process of paying on Uniswap? I'm a bit confused about the steps involved. Do I need to connect my wallet first? And what kind of wallet is compatible with Uniswap? Also, how do I ensure the security of my transactions? Is there a specific fee structure I should be aware of? Additionally, can I use fiat currency to pay on Uniswap, or do I need to convert it to cryptocurrency first? It would be great if you could provide a step-by-step guide on how to make payments on Uniswap. Thank you in advance for your assistance.