The purpose of having a sponsor is two fold, one to have another Orthodox Christian who serves as a witness to your baptism/chrimation who is not a clergyman---this is important when there is a period of persecution and the church did not register its members so lists could not be found to accuse you or convict you. The other reason is perhaps more needed, your sponsor is the go-to-guy to help yopu adjust to your life as an Orthodox Christian. One of their main purpose is to pray for you throughout their earthly and heavenly life. Another one is to demonstrate the orthopraxis or practice of daily orthodoxy (fasting, prayer, attendance at services, bake prosphora, make kolliva, help you learning prayers or practicing the Jesus prayer, etc). If your sponsor is married, he is your godfather and his wife would become your godmother.
It is important to note that in church canons their are limits that are placed upon relationships to your sponsor , to whom you can and can not marry---you can not marry the spouse of your godparent who is widowed, the parents of your godparent, their children, or grandchildren. For these reasons it is important that you select the right sponsor or godparent, especially in a small parish where, if you are single, the options for marriage may be limited within the parish.
Just as your sponsor prays for you, you as his godchild will also need to pray for him. If he is older than you, you as a godchild should be like a son to him. As he ages you will become the one who will pick him up to go to church ,when he has trouble driving. You will be the one who stands and sheds a tear at his deathbed. You will be the one who should sponsor (or co-sponsor with his family) memorials annually for the repose of his soul. the realtionship between a godson and a godfather should be as closer or closer than you have with your blood-father.
I hope you will prayerfully select a sponsor for yourself and enter into that special relationship that God has set apart as an Orthodox Christian convert.