Answer 1

Well i can tell you one thing, you are going to need a lot more then just plain HTML..

For the front-end you would probably want to use: HTML, CSS (maybe bootstrap) and some JavaScript frameworks like JQuery and NodeJS or AngularJS.
And that is just the presentation of your application + user interaction. (This apps provide basic courses and challenges for the things listed above)

Besides the presentation you want to use some logic on your user input. You would probably end up writing code in a language like Java, c# or python. In the code you would create an API that let you communicate between your front-end and back-end. In the back-end you will create a domain, with objects/classes that hold data state in your program. This enables you to use logic on that data. Back-end programming is really complex and a lot of stuff is envolved. I suggest you start learning one of the languages i just listed and then move on with thinking about the next step to create your hotel reservation app. (This app provides the basics for your language to learn. Don't learn swift because it's for apple app development and i guess one step beyond what you really want.)

We also need a place to store your reservations. Such things are called a database. You probably want to create a relational type one, like MySQL. (Take the SQL course in the app for some basics)

PS. Important thing: you may want to make a list about what you want to accomplish with your app. These are called requirements. You can split those in a functional and technical design, which describe your system.

Good luck!