If I relinquish parental rights so that my daughter can be adopted by her step-father, will I be liable for any past or future child support?

Relinquishing parental rights is not something  the courts take likely.   Although not always stated the general barometer is for "good cause."  Relinquishing your rights for adoptive purposes is seen in a better light.

The actual process is called "termination."  This can be done voluntarily.  It may be done in the process of the adoption procedure and you may be able to file a written consent.  You would need to seek legal help in your area with the entire process. 

Terminating your rights relieves you of any obligation to your child from the date of termination forward.  It is doubtful that any jurisdicition would relieve you of past support especially if it can be or is reduced to an order or judgement.