There are two main options regarding housing in Joensuu: private market and student housing.
Getting to the private market usually enables you to choose more exactly the place you will live in, though it is a little bit more expensive. In order to rent an apartment this way, you usually have to directly contact the owner or the house, check for an advertisement on local newspapers or go to a real state management agency. This makes it a little bit more difficult, because usually you have to do it once you are already in Joensuu even though you can check opportunities online. If you want to browse your options online, you can start from Vuokraovi.com.
The second option is to use the services of Joensuun Elli which is a company that manages student housing in Joensuu. You can apply for an apartment online so this may be more convenient than looking for an apartment on the private market. The downside is that you lose flexibility in choosing the place you live in, but student housing is usually a lot cheaper compared to the private market.