The Hosting Plan has nothing to do with the app needing a database, since they are 2 separate Azure services you will be using. You can have an app running in the Free or Shared hosting plan, running on a Premium SQL Database. Or have an Isolated Service Plan running against a Basic SQL Database.
You should be able to figure out, at least get a feel for, what the initial load of the application and database will be. You should start from there.
Important
You're not committing to the Service Plan you choose at the start. It's just to get started, scaling up or down is real easy and fast. So you shouldn't worry to much about the choice you make there.
As far as I'm concerned, you should:
Start small, test often and scale when necessary.
EDIT:
For more information on App Service Plans, see Azure App Service plan overview. Here's (very) brief summary when to choose what, based on How to choose an Azure App Service Plan:
Free
Use this plan for PoC & development purposes. Production scenarios to be avoided for sure as you don’t have neither SSL nor a custom domain support
Shared
Though this tier is one level ahead of free tier, it's suggested only for hosting basic apps where you don’t have any security or recovery options as this tier also doesn’t support SSL and also backups.
Basic
This is the starting level tier for your production workloads. You get a dedicated instances & with 99.95% SLA, SSL support to put your apps to real work. With multiple instances you also get your app load balanced.
Standard
This is a pure fit for your serious production apps with a IP based SSL support.
Premium
As the name suggests, use this for your premium/high intensive critical mission apps
There's a lot of information available, listing it all here wouldn't add much to the answer.