There are a number of different routes you can go depending on how you want to help and how much money your want to make. Working for a prosecutors office will allow you to advocate for DV and sexual assault victims (along with victims of many other types of crimes). You'll want to work for a county prosecutor's office if you want to handle sexual assault cases since most sexual assault charges are felonies. You can also look to work for plaintiff's law firms that focus on representing crime victims. Finally, regardless of where you work for your paycheck you can always find pro bono opportunities to advocate for victims who need help with things such as landlord-tenant disputes, fighting disclosure of personal information (e.g. health care records) to defense, and immigration.
If I understand your question? First you are asking if there is anything special you do or take during law school in prepare you for the field of criminal law. No, you will take criminal law and criminal procedure in law school and you do your very best and graduate and pass the Bar. During law school you can try and intern at a prosecutors office somewhere. Most offices will have deputy prosecutors that deal with DV cases all the time whether it is misdemeanors or felonies. You get your feet wet in District Court and then maybe get some experience in Superior Court. GET READY TO BE ON THE DEFENSIVE WITH VICTIM CLAIMING SOMETHING OTHER THAN WHAT THE POLICE REPORT SAYS. Realize that there are victims that do need help and are victimized by thier significant other HOWEVER there are some victims that just lied to the police for many many different reasons, of which I have examples if you want to hear about them. Good Luck
Go to http://dev.wacasa.org to get information on how to become a CASA (court-appointed special advocate) volunteer in your area.