Short answer - Anywhere in the city centre works. Warsaw has hundreds of accommodation offers from hostels to 5-Star hotels.
Example search on booking.com:
Hostel: 14 euros per night (single bed)
2-Star: 33 euros per night (twin)
3-Star: 45 euros per night (twin)
4-Star: 55 euros per night (twin)
5-Star: 85 euros per night (twin)
If you need some help, we would be delighted to offer some guidance.