I am a licensed canon lawyer.
Not enough about your situation is presented for anyone to offer a useful response to your precise situation beyond what stbruno has said, and I think it would be negligent for anyone to try to do that here. May I simply second the recommendation that you contact your parish priest today, discuss the situation thoroughly with him, and proceed from there.
We can only speak in these general terms.
First, Catholics are normally required to marry according to the canonical form of marriage, i.e., to give and receive marital consent before an authorized Catholic priest or deacon and two witnesses. This requirement is imposed for validity. In some cases, exceptions apply.
Second, Catholics cannot, even if using canonical form, validly marry an unbaptized person without a dispensation (a relaxation of Church law by a competent authority) from what is called the impediment of disparity of cult. Again, in some cases, exceptions apply.
Finally, it is unusual pastoral practice for a person to become Catholic by either baptism or reception into Church when there is a question about the validity of an existing marriage.
Your parish priest will have had the kind of training required to ask the right questions and to assist you in this matter.
Evaluating questions of mortal sin considers not only the objective nature of an action but also the knowledge and intention of the person. This belongs to your parish priest as confessor and counsellor.