How do I build an app?

[Directorofmishaps]

I want an app which goes alongside the website I have already. Fairly basic. Will need to be able to add things and also for front end users to be able to search the things added.

How do I build an app?
How much does it cost?
Where do I start?

Can anyone help please?

Why do you need an app rather than a responsive website that will function well on a smartphone? If its just to search and find information then just make sure it works well in a smartphone browser.