～から、～ / ~, so ~

AからB

means "A, so B.". A is the reason for B. At first, see what sentence for A.

きょうは　こどもの　たんじょうび　です

"きょう/today", "こども/child", "たんじょうび/birthday". so it means "Today is a birthday of (my) child.". We will put this sentence in A as a reason. For the second, we think about what to put in B.

はやく　かえります

"はやく/early","かえります/go back home". it means "I go back home early". This sentence is for B. 

きょうは　こどもの　たんじょうび　ですから、はやく　かえります。

this is a long sentence. But don't afraid. this is "AからB / A, so B" grammer.

「きょうは　こどもの　たんじょうび　です」から、「はやく　かえります。」

A is a reason, B is main action. It can be translate as "Because A, B.". So, this sentence means "Today is a birthday of (my) child, so I go back home early.".