In case you owned some BTC you can exchange it easily for Ardor in one of the exchanges who manage a BTC\ARDR market.
for example:

 1. Pholoniex.
 2. Bittrex.

In case you don't own any BTC you need to buy some and then the above.

In general:
before trading you need a platform which allows you to keep ARDOR tokens and let you commit transaction on the blockchain - it can be on web exchanges, desktop client or else.

When this is done (you found a place to store your tokens), you can search for exchange place who have a market Ardor\AnyKindOfValueYouWant and there you can start trade with users.

By market I don't mean only one of the listed above, it can on be even on instant msg platform such as Whatsapp, Telegram or Facebook group.