Most people who are interested in building their own app have two main considerations in mind. First off, they know very little about coding in general and want their app builder to do all or most of the work for them. Second, they want to build their new app for free or at a very reasonable price.
This generally means that they want to start out with a very simple app and then maybe tackle something grander down the road. With those parameters in mind, people might want to check out the following best app making sites:
Appy Pie seems like a good choice for a first app builder. Plans are free and scale up from there. Simple to use. Not for complex apps.
IbuildApp gets you going in as little as 5 minutes and comes with a free trial so you can see if it meets your needs.
AppMakr has built a ton of apps for people and includes a simplified free plan.