What is your mobile model?To insert simcard,the following conditions should be satisfied....
1)The mobile should be unlocked to accept the sim card
2)The mobile should have the sim slot (most CDMA phones do not have that provision)
3)The simcard size should be supported by mobile phone.
Now,if your mobile is locked with a particular network then you need to unlock it.For that you can either approach the respective network provider or third party unlocking services like http://www.prounlocking.com/supported-phones/rs8wp... and unlock your mobile.
Once your mobile is unlocked the sim slot will either be in the side or above or below the battery depending on your mobile model.If you search in Google you probably will get a more clear idea about where your sim slot is located.Once you find the sim slot remove the sim card and note down the position; replace it with the old sim card(it should support the phone sim size specification).
Regarding the sim size there are many options like cutting the sim or use the apt sim adapter hardware that is available in the market.If you are going to cut the sim you can get it to a professional or follow the free Youtube video available.If you are not comfortable with any of the above you can approach your respective service provider and order for the desired sim size.