Is polling the only way for updating app’s data from a database

First you will have to use query language. All the flavors of SQL use the almost same SQL query syntax. So you will first have to learn SQL query language. For eg. you can start with tutorial at www.w3schools.com/sql/default.asp

Stanford University had offered public course on databases last Fall. They had nice tutorial on how to set up SQLite.

Once you start learning SQL you will need some tool using which you can build a database and run queries. SQLite is free software library which can be used to create databases and run queries. It can used as an independent tool i.e server setup is not required for using it. It also supports importing csv files.

Once you are familiar with SQL, you can use MySQL if you want to design some web-application or access to build desktop applications. I do not think Microsoft Access is at all necessary (as it is not free) if you just want to get started with.

I would lean toward multiple databases. Primarily because you can set them up for different recovery, SLA, maintenance, deployment, changes, etc. And also this allows you to move certain databases to faster or larger I/O on different drives without disrupting everyone.

For the master tables that are used by all the applications, no I don't think you should have multiple copies of these. Your applications can query different databases and/or have different connection strings for different tasks (best separated by a middle tier of some sort in most cases). If the app-specific code within a single database needs to have some kind of reference to them, you can use synonyms or three-part names. This is easy to extend if you later need to split out not just across databases but across multiple instances / servers (just add a linked server and make it a four-part name).

Only if the link between these servers is notoriously slow or otherwise unreliable would I consider making replicas of the master tables. I talked about how to do that here and here, even though I don't explicitly cover moving data across servers.