The Amsterdam Conference Center is a standalone conference center and not a hotel. Located in the city center, however, there are many hotels in walking distance. You can also stay elsewhere inside the city, the Central Station is a 5m walk so you should be able to get around easily (and trams 4,14 and 24 stop right at the venue). Anywhere inside the canal area, plus the southern hotspot neighbourhood De Pijp, as well as Europaplein are reachable within 30m with tram or metro. The latter with the newly constructed ("north-south") Metro 52 which stops just south of Dam square and also takes you across the water to Amsterdam North within 10m. You may also consider renting a bike, as this is generally fastest to move around (bicycle traffic is pretty dense, though, so you must be comfortable with that).
Please consider booking your hotel room early, as soon as you decide on attending. SIGMOD/PODS 2019 falls inside the touristic high season, so rooms may become scarce and expensive when booked close to the conference starting date.
The conference venue is offering a hotel service that will hold on to blocks of reserved rooms at fixed prices. However, we also recommend first checking the usual booking sites, because these may have better-priced offers still (e.g. google, trivago, booking, expedia, airbnb, ...)
Tips: if you only find expensive options, here are some good yet inexpensive hotels reachable by public transport (in order of our personal preference):