How to transfer money to BTC markets?

Could you please elaborate on the process of transferring money to BTC markets? Are there specific steps that one must follow? Is it a complex procedure or relatively straightforward? Additionally, are there any fees associated with the transfer, and if so, how are they calculated? Furthermore, are there any security measures in place to protect the funds during the transfer process? Lastly, what are some of the most popular BTC markets that users typically transfer funds to?