Forgive my reinterpretation of your question, but:
"I want to create software for employee scheduling and management, something which is typically only provided by teams of experienced developers over a span of a few years. Also, I want it to be cross-platform. My skillset includes MATLAB and opening a Java IDE. Where do I start?"
Uncharitable? Maybe. but there is such a thing as biting off more than you can chew. And the breadth of this query is- well, a bit silly!
1. Create a design document from the perspective of the user. What capabilities does it provide, How will you, as a user, use the software? For software like this around 50 pages would be terse but workable.
2. Create a technical design document drawing from the first design document and describing how you plan to implement those features.
3. Implement the features in the technical design document.
Just as an overview- It's obviously going to use a database. Nobody can tell you what database to use- But I will say that MS Access would be a poor choice. Postgres is pretty good. You'll have to design the tables and you'll have to know how to work with a relational database if you haven't already as well as pick up on the query language.
Since you want to support Apps, it needs to be possible to interface with that database via HTTP. This would mean learning how to program server-side web pages. So you'll have to choose a server side development technology and implement a database layer. realistically, you'll want all the business logic which deals with actually manipulating and dealing with the data as part of the web API itself. So that will be where the bulk of everything happens. Client apps/applications would interface through that web server to read and set information. For example perhaps an employee can book time off or something and that saves that information to the database which is used when constructing schedules.