Finding accommodation in Warsaw shouldn't be very difficult. There are many options available and the only criteria is how much money you are willing to spend.
Flatsharing is definitely the cheapest way of finding accommodation. It is especially popular among students and young professionals.
In case of partnership, people who have signed the lease are the only legal tenants, and therefore are responsible of the accommodation (it is possible to let the landlord know of this partnership if officialised by the town hall for example).
There are a few good sources of finding accommodation for your stay.
This is the best choice if you want to find accommodation quickly and without stress. However it is also more expensive way, as real estate agents will usually charge you a fee or add it to your rent.
For the most comprehensive list of real estate agencies in Warsaw and region visit http://www.oferty.net/
Update 6/02/2015