All you need to do is call your service provider, write the numbers down on the phone and battery. It should be easy unless it is around 5 or more years old, in which case you may not be able to legally do it.
The main thing involved in activating, any mobile phone for that matter, is to see that the battery is valid for use. If not change the battery and charge it. For usage you must have a sim card from any of the mobile service provider, preferably of your neighborhood.
To activate an old mobile phone you'll need a new, working SIM card, compatible with the handset. If you don't already have one you can get a Pay As You Go one. Charge phone, insert SIM, enter the given pin and follow instructions.
Mobile phones are tied to the company that sold them. Usually all you need to do is contact that company and set up a new account. However, older phones may no longer be compatible with some networks, so may not work at all. Call the company to find out.
First of all make sure that the phone dose not has any technical problem. If yes, give it to a mobile phone mechanic. Then charge the battery. Then use a sim card in to the phone