lalitmishra1988, before replying, I want to say only you will ultimately be able to determine the needs of your app. The below are only suggestions and should not be taken as consulting due to the number of factors that can impact the needs of your app.
With that being said, this sounds like a production app. We typically only recommend production apps to be run on standard/premium/isolated tiers. You can see the availability SLA along with the other attributes here. The basic tier might be able to run your application but if you're concerned about availability, you might want to stick to standard tier or higher. If you only plan on running a single site on an instance, you'll likely be okay with something like a standard small (S1) instance. If you have a resource intensive site or multiple sites in your app service plan, you might consider a medium (S2) or large (S3) instance.
You also might want to consider scaling out vs scaling up. What you talked about, basic/standard/premium is in regards to instance size. If your concern is having a high number of visitors, you might want to consider scaling out by adding more instances to your App Service Plan. This is the better approach to handling large amounts of requests rather than changing your instance size. While this blog is older, it talks about the basic concepts of when to scale up vs when to scale out.
Since you mentioned high availability, one thing to be careful about is to not think that having two instances in the same app service plan will provide redundancy. While unlikely, if the network goes down in your data center, both instance will be unable to communicate with the internet. To resolve this problem, you would want to consider using Azure Traffic Manager (ATM), which is a load balancer that can direct traffic to multiple apps in different regions. You in theory could create a West US app and East US app and then use ATM to route traffic to whatever region is available. This option is best for mission critical sites or store fronts that could lose money caused by a single instance outage. For more information on ATM, this is a good place to start.
Let me know if you have any further questions or concerns.