Could you tell me more about what I should focus on the most?
There are specific professions with high chance to find a job later on. For example taking care of elderly people (Altenpflege) is a field with high demand for labour. In the financial business the chances to earn a lot of money might be higher.
You always have to take into consideration in which area you are living. Berlin for example is not an industrial city - it has a focus for services.
There are basic tasks that need to be done in every city: for example you have to provide electricity. When you do an apprenticeship in a field like that than you can work everywhere in Germany. Event if you only have a Duldung and you are send back at some point of time you can profit from your acquired skills. You can use them in your home country.
Broadly speaking, there are so many professions that you can do an apprenticeship in that everyone should find something that fits their talents.
There is a positive-list of the Bundesagentur für Arbeit that contains all the professions that have a high demand for apprentices. For example work with renewable energies or caretaking.
As Robert said, keep in mind where you are living: in Berlin there is no heavy industry, for example. If that is what you are interested in then you probably have to move to different parts of Germany.