How to Get to Manaus:
As mentioned, the Port of Manaus is right in the downtown area, so once your ship gets in it is as simple as walking into the city center to enjoy the sights. Not all the attractions are downtown, of course, but from central Manaus you can take a bus or taxi to outlying destinations you might want to visit - like some of the river beaches or museums.
One Day in Manaus:
Because central Manaus is within walking distance of the port, it is easy to spend a day exploring the city center on your own. You can visit the beautiful Teatro Amazonas opera house, pick up some souvenirs at the Mercado Municipal and learn about the indigenous populations at the city's history museums. Getting to points that are further away, like the Natural Science Museum or the popular river beaches, would be easier with a tour so that you don't need to figure out transportation or worry about getting back to your boat in time if you are visiting from a cruise. If your stop in Manaus is at the end of your cruise, you can also book an Amazon tour. The two rivers that meet at Manaus form the Amazon, and boat tours leave regularly from the city.
The local language in Manaus is Brazilian Portuguese, and the currency is the Brazilian Real. You will need to exchange your money for the local currency, and be prepared to have smaller bills - many vendors cannot make change for large denominations. Note that although the port is within walking distance of central Manaus, the weather is typically very hot and humid, so be sure you have sunscreen and maybe a hat to protect yourself on your walk.