Your trip will most likely start from a big transport hub, like the airport / railway station. There should be free wi-fi in case your home service provider doesn't allow you to connect to a cell network in Moscow.
We recommend to visit an official shop of any Russian mobile network (
MTS,
Megafon,
Beeline,
Tele2) and purchase a SIM-card. You will need to present your passport. The probability that staff of such shops in airport or in railway station speaks English is higher than anywhere else.
In order to use the internet on your first day in Moscow you can use public Wi-Fi. Most streets in the city center, parks and almost all public transport provide free internet access. Wi-Fi networks are available at over 1,000 locations. Each of them will be marked accordingly to keep you informed. To connect, select "Moscow_Wi-Fi_Free" among accessible networks, open the browser and register. This can be done through text messages; you should enter your Russian phone number, and receive a text message with a code.
We also advise you to bring a powerbank with you (most airlines and railway carriers allow you to carry a 20000 mAh powerbank).