You need information first on where to go. There are many agencies across the country , and some even cover international adoptions. We got three of our children from the state adoption agency in Massachusetts and two from the Catholic Charitable Bureau. You want to find out what agency you should go to. Some charge more than others, promise you the world, and put you through a third degree. You don't want to have to put up with the added aggravation, so agency selection is important. You want to be able to evaluate different agencies, and make your own decisions. You want to find the states with agencies which are the most "adoption friendly".
You need to understand the application process, and how the home study works. I remember the first home study we had, we had a small house with a basement which was prone to flooding. The day before it had rained continually, so the basement was covered with water. We thought we would never pass the home study under those conditions! We borrowed two or three fans and let them run all night, hoping against hope the basement would dry out. Luckily the home study worker was afraid of dogs, and we had put our dog down there. She never even went into the basement! You will want to know what to expect from the home study.
If you are considering adopting older children, you must realize this is an entirely different situation than adopting a baby. I noticed my girls (three of them) had some antagonism toward me, which was lacking in the boys. The adoption worker told me that this is very common. Possibly because boys lack the "maternal instinct" and don't feel as strongly as girls do about their mothers' having placed them for adoption. So they have less resentment toward the "mother figure" in their new family.
If you are interested in international adoptions, chances are you will have more questions, including travel requirements, what to pack, exactly what this "journey of a lifetime" requires. An experienced agency should know what to tell you in this regard.
I often thought it would be nice to have someone leading us by the hand. Someone who has "been there and done that", combined with other professionals who could answer all of our questions. Wouldn't it be great to have a network so you wouldn't feel so isolated? Our first adopted child was a five months old boy, and we waited nine months for him. (I couldn't have done better myself!)
The very best of luck in your adoption journey. It is an experience like no other, and I am so glad I completed that journey six times!