How do you charge cell phone in Europe ?

Most chargers made these days are dual voltage, check your charger. If it isn't, then buy a dual voltage charger for your phone make and model before you leave. Check the internet or the website of the manufacturer to purchase.

You may need an adapter if the charger is a US model with 2 flat pins. Some chargers come with adaptors as well.

Other than that, it is just like charging in the US, plug in and leave it. I have charged from shaver only plugs in European hotels as well as regular wall plugs.

Thanks,
this was brilliant "plug in and leave it"

I think my charger is 2 flat pins.
So, can I just go to T_Mobil store and ask for dual ?

First check to see if the charger you have is dual voltage, that way all you need to do is buy an adaptor. If you bring it to T-Mobile, they should be able to tell you if it is dual voltage; also look for 110/220v on the charger itself, that means it is dual voltage. (Also could say 220-240/110-120 or variations on that arrangement)

You can get adaptors in most drugstores and hardware stores in the US (as well as in Europe), there is no special adaptor for a phone charger.

What European countries are you going to? Different countries have different configurations, i.e. UK has 3 pins in a triangle, Switzerland has three pins in almost a straight line. Useful websites for plug configuration info are http://www.voltagevalet.com/country.html or traveloasis.com/elad.html