New Girl

Hi Guys,
I have just joined this site and I am very excited. I do have one question for now...If you work out do you always eat back your calories that you burned through that work out. Example: my daily calories is 1200, workout calories of 300. My net is only 900.
I am afraid if I don't i will put myself in starvation mode. I think the rule if thumb is always let the net calories be the amount in my setting of daily calories. If not, eat them back...am I right? Also, would love to meet some friends. I have about 70 lbs to lose!
Thanks anyone!!