I didnt know if there was a list somewhere of the more popular BTC exchanges. I have been searching. I found Kraken. It appears to be laid out nicely. Id like to look at a few that allow trading in smaller amounts for someone just starting out.
I would take the suggestion already given here and go to coinmarketcap and see what are the exchanges with more traded volume on the pair bitcoin/(alt you want to trade). I would read their TOS carefully though, before making any deposits, since most of them are getting regulated and might need a license to operate etc, and it's possible they will start asking for ID verification. It is also possible that they might have to close, so also see in what country they are registered in, and how that country sees bitcoin.