As you live in a flat, a burglar alarm should be quite simple. You obviously need the entry door protected with a contact switch and one or two sensors - i suggest in the entrance hall and protecting your computer equipment.
A colleague at work had an alarm installed that telephones his mobile telephone if it is activated.
Will anyone hear the alarm or take any notice of it?

I think you probably should increase physical security first.
Consider an additional mortice lock on your front door or possibly an iron gate/security door. Sometimes this is the only way to stop burglaries.