I second everything that the previous posters already said about making sure your marriage is stable before you start any adoption proceedings. Even if you decide to begin the process, there is a pretty detailed homestudy with questionnaires, home visits and at least a couple of interviews of you and then your DH too. The social worker will be looking to see that your marriage seems stable and that any child placed in your home will be loved unconditionally and supported as he or she grows.
You say you don't know where to start. First I would do what I could to make sure your marital relationship is strong and fulfilling before proceeding. I understand and sympathize wholeheartedly with your lifelong desire to become a mom, but I hope you are not seeking to heal a wound caused by feeling second-best in your marriage by adopting a child who will love you unconditionally. It wouldn't be fair to the child.
Your age is not a factor for most adoptions. DH and I are 44 and were contacted by several potential birthmoms. And I have single women friends who have adopted domestically and internationally.
I hope I didn't sound harsh. That wasn't my intention. I sympathize with you as I was in a troubled marriage years ago and I remember how exhausting and demoralizing it can be trying to figure out how to make things right.