I HAD A CHILD BY ANOTHERMAN WHILE STILL BEING MARRIED BUT WE WERE SEPARATED, WILL THAT AFFECT ME WHEN WE GO TO COURT FOR THE DIVORCE?

California is a community property state - the only time having a child will impact you in a divorce in a community property state is if your husband has been providing for this child, claiming it as his own at any time during the marriage, whether you were separated or not.   If he is on the birth certificate, and/or provided for the child, you, he, and the court, will need to reconcile the statutory presumption that this child, born in a marriage, would be considered his.  If he has not provided for the child and is not intending to, and the child is not his, it must be made known to the court that he will not provide support for the child.  Since California recognizes legal separation, any costs incurred for the child while you were separated would be your debt and not community debt.

California ia a "No-fault" State, which means either of you can file for divorce, and adultery and children not of the husbands are not considerations in a no fault State.  If you are certain the child is not your husbands, you must make that fact known to the court.  If it is not certain, then a paternity test ordered by the court would reconcile the issue. 

Affect you in what way?  As a ground for divorce? In the distribution of your marital assets?  It should not affect anything.  California is a no-fault divorce state - in fact it was the first to implement it - and  California is a Community Property state and marital property is split 50/50.  The Court does not look at who did what to decide anything. The court does not even consider fault in deciding spousal support issues.  What I would be concerned about is that children born during a marriage are assumed to be children of the marriage.  So that issue needs to be addressed satisfactorily for the court in order to move past child support issues during the distribution and support portion of the proceeding.  Good luck.