Start with an OB/GYN then get a refferral to a fertility specialist.
Most of the medications you would need a standard OBGYN can prescribe and monitor. Be prepared for lots and lots of blood tests.
The next thing to do is get as healthy as possible. Do Pilates to keep your abdominals strengthend. Stop alcohol, and smoking if necessary. Cut back on sugar or adjust your diet to keep your hormones in check. Most of all spend a lot of time with your little one because once another is brought in that quality time is cut down to a third.
For me it was that I wasn't ovulating. We had been trying for 2 years. The first thing they offered was rebound ovulation. I took birthcontrol pills for a few months then I was supposed to ovulate the next month off of them. Well it took a few more months to concieve DS#2 but after that I was getting pregnant left and right.
I did the same thing when I got pregnant with DS#3 this time I was pregnant the very next month.