It depends on what your server side should be capable of
First thing you can do is determine the functionalities by talking to the other coders, the designers and the producer. Even if you are in a smaller team where this roles mix up you can identify needs in terms of game play experience, business model and app performance. If you know that, you can decide (with the team) if you build the whole stack yourself or if you can use (cloud) services. You can even go down to setup our own physical servers in your office but this probably means that you cannot handle a lot of traffic (remember: app performance).
Nowadays building a backend can be more like assembling than actually building everything from scratch. It starts at the bottom, the infrastructure layer, where you can go for Infrastructure-as-a-service offers (Iaas) like Amazon Web Services or a layer higher at Platform-as-a-Service solutions (Paas) like Heroku, Mircosoft Azure or Google App Engine. At this point it´s also important to think about which kind of data you want to collect, how you want to store and access it, speaking of data base system and data ware house because Iaas or Paas providers not always support every kind of data system setup.
Still, you might want to keep the effort for all this in mind. You can build the most sophisticated system ever but if it exceeds the requirements its normally a wast of valuable engineering time. So, if your backend requirements can be nailed down to a specific set of features you can also go for a Backend-as-a-service solution (Baas) or even the offers of certain app service providers. The Baas solutions available out there differ and you want to find the one that´s matching your use case. Here it´s gaming, so you might want to take a look at NativeX, Playnomics, Applicasa or OpenKit for example. If you want completely spare the effort of doing any network programming, there are app services like swrve or playhaven who go for a special set of features.
If you go down this latter of building you own stack, so all levels of your software architecture, starting at infrastructure level up to the point where you are "only" integrating an app service via SDK or API always keep the road map of your game project in mind. If you want to have a special level of insight or control later in the games' life cycle make sure that your backend setup can do this because exchanging it ones your game is live will cause massive effort and you might lose users alongside the process which in a mobile game means they might never come back again.
Keep in mind: All this services and solution (can) have a price attached so make some calculation and get the OK from the management before building something in or at least before something goes live.
Hope that helps ;)