What to expect when searching for a private flat:
Beware that most of the times as soon as the landlord or the agency posts the ad, they will receive many calls and this reduces your chances of securing the apartment. Also, when you will be given the time for visiting the apartment it is possible that you will have to wait for your turn since the landlord have given the same time to many perspective tenants.
Most of the times being foreigner is a disadvantage since you don't speak proper German or you don't have a "believable" proof for payment. You might be asked for your finances, how are you planning to cover all the costs and how much is your monthly income. You should be prepared to show more or less 1000 euros/month income. Sometimes you might be asked for someone to guarantee for you, that in case you don't pay your rent or bills this person will cover your costs. Normally this would be your parents, but sometimes landlords prefer having someone from Germany guaranteeing for you.
Do not be discouraged. This is not always the case. Sometimes landlords are really flexible in their requirements, and sometimes they might even prefer a foreigner to a German person due to the internationality. This especially implies for WGs (shared apartments).
When an ad is from Makler (real estate agent) or Verwalter (service provider) you usually pay a commission (Provision) of 2 monthly rents, or most of the times 2.28 * KM (Kaltmiete = cold rent). Sometimes they might decrease the commission, but in any case they can be really helpful if you are in real hurry to find an apartment.
Sometimes in the ads you will see something like 3.5 room apartment. This half room is actually a room in connection to one of the other rooms, and normally it doesn't have windows and is too small to be considered a full room. Normally it is used as a storage room. When apartment is advertised as a two-room apartment it means that it has one bedroom and one living room. Kitchen, bathroom and hallway are not counted in the room number. If you want to rent an apartment with a friend, landlords might consider that a two-room apartment is too small for two people although you think it is good enough for you.
Prices of the apartments will vary on the location and the size of the apartment. Most of the time the price is determined by the size of the apartment. Rooftop units, where the hight of the room is between 1m and 2m are calculated only as 50% of the size, so they might appear bigger in reality. Apartments with balconies are generally more expensive, and the more central the apartment is, it's price gets higher. Generally the prices are listed as KM (Kaltmiete = cold rent) which means in addition to this you will need to pay NK(Nebenkosten = additional costs) such as garbage disposal, building maintenance, water, electricity, heating, etc. Sometimes the rent is Warm (includes the utilities). Approximate price of a two-room apartment is around 450 euros cold + 150 euros for utilities but this varies. For the same price, depending on location you can find a three-room apartment. One person apartments are usually smaller two-room apartments and can be found for as low as 250 euros + utilities or maybe even lower. But do not underestimate the utilities value. Sometimes you might need to pay low amount for utilities per month, but after some time, usually ones a year, there will be a recalculation of the expenses, and if you are lucky you might get some money back, or if you are really unlucky you might have to pay additional few hundred euros for the extra spent. Thus be very careful and try to save on energy.
Please note that the landlady/landlord asks for a deposit (usually one or two months rent), which you will get back when you move out again (and no damage is done to the flat, of course).
Möbliert oder unmöbliert
Apartments are generally rented as unfurnished, half furnished or fully furnished. When they say unfurnished, they really mean unfurnished. You will not find anything in the apartment sometimes not even a lamp. Since most of us are coming here for relatively short time, it is advisable that you get an apartment that at least has a kitchen installed since this is not cheap to do on your own.
As an alternative to an apartment you can get a room in a WG (Wohngemeinschaft; shared apartment). These are shared apartments and it is a cheap alternative since you will split the costs with the others in the apartment. It is also a good chance to meet new people. Usually WG's are shared among students and young professionals, but there are also the ones for 50-60 year olds. In the ads for these kind of rooms they usually give the ages of the people already living there. Living in a WG means that you will share the bathroom and the kitchen with the others who live there. Generally WG-Zimmer (room in a shared apartment) is unfurnished, or has only a bed.
Usually the price for a room in a WG, depending on the size is 200-350 euros/month cold.
As a temporary accommodation or a sublet (usually for couple of months or even a year) you can find a room rented by a tenant rather than a landlord. This usually happens when one of the tenants of a WG is leaving the country or the city for some time. These kind of ads are usually labeled as Untermiete or Zwischenmiete. Most of the time this kind of rooms are rented furnished, and you might have to pay a deposit to the previous tenant for his/hers furniture in the room.