Dominican Republic's currency is the Dominican Peso, DOP.
It might be possible that Canadian dollars are accepted, but I suggest to use US dollars (USD), as they are widely accepted. In touristic areas (and resorts), restaurant menus and stores have their prices in USD. You can use either USD or DOP, they will be both accepted most of the time in touristic areas.
If you are changing currencies, most resorts have currency exchange, so you can do it inside the resort. Contact the resort you are visiting for that matter or they might already have that info on their website. I don't know about Punta Cana airport, it is possible they have an office of one of the local banks and you might be able to do the exchange there, but I don't have experience on that. 1 USD is about 43 DOP (as of may 2014. Google has a currency converter, you can check the query "1 usd to dop").
About tipping, I'll quote this:
Restaurant bills already include a ten percent tip. It is customary to give an additional 10% for good service. Most people do not tip taxi drivers, however if you feel so inclined for good service, a tip will certainly be appreciated.
Source (and you can also find more information on other topics in this link):
And I'll add, apart from restaurants, it depends on the city you are because some cities are higher cost living than others, but 30-50 pesos is an ok to good tip. 100 pesos is a very good tip. If they gave you a good service, feel free to tip.
Edit: link and quote were updated at godominicanrepublic.com. Reflected those changes here.