Yes, it's a cliche to say: "start at the beginning and complete each step until it is finished."
But, that's also a profound truth; but, first, you need some kind of a plan ... to locate where the beginning is. How do you get a plan:
Start by doing basic research. Search CodeProject and the web for on-line testing/exam software, or software, like slide-show, or wizards, that present a sequence of something.
At the same time, work on outlining a set of features your software must have; make notes as you study what you find on the web, or by looking at how other programs work. Sketch ideas for the UI.
You will need to think about, and plan for, the use-cases of your software: think about things like "scale:" how many users at one time (a few, a hundred, several thousand). You will need to plan for how your on-line application does, or does not, persist data: that leads to consideration of security (users, accounts, roles, groups, passwords, etc.), and what database, what type of database, you need to use.
You will need to choose software libraries to use to give your web-site functionality/facilities, and to what extent you will locate code and resources server-side vs. client-side.
To avoid getting overwhelmed in information and possibilities, I suggest you start early to create a simple web-site prototype to simulate presenting exams, and responding interactively to a "mock" user.
The journey can be fun if ... you take it carefully, and slowly.