Welcome to Android Central! Who's your carrier? Have you asked them about this? Although conference calling is pretty standard for basic wireless plans, it's possible that it might not be included in very basic prepaid MVNO plans.
I can't help you if you don't even know which wireless carrier you're using. Verizon? AT&T? T-Mobile? Sprint? Straight Talk? Cricket? Tracfone? EE? Three? Vodafone? This is pretty basic information for any cellphone user.