Here, we will provide you with some general information about Berlin, Germany and Europe.
Electricity in continental Europe is in general 230 Volts (not 110 Volts like in the US and other countries). Powerplugs are also different. So, make sure you have enough adapters with you. If your chargers (for laptop, mobile,...) do not support 230 Volts, you will also need a power transformer.
Europe has a different mobile phone infrastucture than the US. In order to be able to use your mobile phone in europe you will need a GSM compatible mobile that is able to work on 900MHz and/or 1800MHz. There is the possiblity to buy a GSM phone in Europe with prepaid phonecards for less than 50 Euro.
Berlin is a quite secure place, normally you don't have to worry about crime (like robbery,etc..). Even during night hours you can walk around everywhere in central Berlin without being at great risk. Nevertheless we all know there can't be a 100% security. Like in all bigger cities in the world, there are pickpockers, conmen and crime like that. So watch your stuff.
By german law, everyone older than 16 years has to have an IDcard / Passport to carry around at all times in public. Police is allowed to ask you for your ID at all times. Police is not allowed to search you and/or your belongings without a good reason / suspicisness (reality shows that law enforcement can be very creative in finding reasons). Police is required to show you their ID upon request, you have the right to write down the name and/or the ID of the policemen that stops/searches/whatever you. If you get stopped/searched/... by police you just have to show your Passport / ID, as well as tell them the stuff stands in your passport (like name, birthdate,...) and a rough jobtitle (like student, worker,...). Do not tell them more, like in other countries: "Everything you say, can and will be used against you". You are always allowed to call a lawyer you trust. You are always allowed to have 2 phonecalls (be aware that police can refuse to change money for the phonecalls, so small change in your pocket is a good thing). Police can arrest you without a good reason for (maximum) until midnight of the following day (this can't be more than 48 hours). Otherwise (if they want to arrest you longer) a judge has to confirm this.
Drinking alcohol and smoking in public is allowed. You have to watch out for "no smoking" signs on trainstations/airports. Other drugs (except tabacco and alcohol) are not legal. Be aware that police is often searching people going from the netherlands to germany (because of drugs).