What’s the best way to start building a D&D character-creation app?

This sounds like a great fit for a web app. Start with HTML, CSS, and JavaScript. HTML defines the structure, CSS controls the appearance, and JavaScript handles interactions such as changing ability scores, selecting a class, and updating calculated values. You don’t have to manually draw every visual or repeat an entire document structure for each character; you create reusable elements and update them with JavaScript. Once the basic version works in a browser, adding mobile support is much easier.

A useful progression would be plain HTML, CSS, and JavaScript first, followed by a framework only when the basic version starts becoming difficult to organize. You don’t need a specialized application editor like a game engine. The browser and a code editor are enough to begin.

Plan the data before building every screen. For example, define what information a character contains and keep the rules in structured data rather than hard-coding every value into the page. Then create a simple form, display the resulting character sheet, and add persistence with browser storage. Later, you can introduce TypeScript, a frontend framework, or a backend database if the project actually needs them.

You could also begin with tools you already understand. Since you have some C++ experience, a desktop UI toolkit such as Qt would let you build a traditional application while staying in a familiar language. That said, web development is probably the most practical direction for this particular project because character sheets are mostly forms, data, and displayed information.

Python is another reasonable choice, especially for modeling the data. You could represent characters, classes, races, abilities, and equipment as objects or structured data, then use a web framework such as Flask for the application. The main difference is that Python would usually handle the server-side logic, while HTML, CSS, and JavaScript handle what the user sees and interacts with in the browser. For a first version, you can avoid a separate backend and store sample data directly in JavaScript or JSON.

Don’t spend too long deciding. Start with one tiny feature, such as entering a character name and displaying one ability score, then gradually add selections, validation, and saving.