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 Simpleunlocking.com
(This is specifically for
Blackberry phones) 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