Learn German

For a short-time stay like that, you need temporary accommodation. Hotels or B&Bs are the obvious choice and cost €30-100/night. Short-term apartments or rooms in shared flats can be cheaper, but much more difficult to find.
For daily expenses, budget €20-30/day for a simple, cook-yourself lifestyle without travels or luxuries. If you want to eat out occasionally,, have some entertainment and sightseeing, you might need to double this.
An intensive German class is advisable in your case, which are offered by some universities, community colleges ("Volkshochschulen") and private language schools. They start at around €300-500/month (for several hours of classes every weekday).
There are many other possibilities for activities where you can practice your German, from sports clubs to guided tours to part-time jobs. The cost obviously varies widely.