A couple of notes:
1. I'm doing this out of order. The next concept on the list was "Coaches, Not Styles", but I actually think the question of Sacrifice needs to be addressed first, since it ties directly into figuring out how you're getting to your goals.
2. I originally blocked out Timeline-in-Training and Sacrifice as two separate concepts, but upon further reflection, they're really two sides of the same coin. One side asks "what CAN you commit to this?", while the other ask "what WILL you commit to this?" Answering those two questions can put you on the right path to planning your training, and looking for a school that suits your needs.
3. Someone asked about age. Age is part of Timeline-in-Training. A twenty-year old who wants to be a UFC champ has a different timeline-in-training than a forty-year old who wants to be a UFC champ. Consider what your body will actually allow you to do. I might do a whole post on this.
What CAN you commit to this?
To answer this question, and the next one, take an inventory of how you spend your time. ALL of your time. That includes work, sleep, meals, hobbies, TV watching, or whatever. Regardless of how you value it, you are doing something every minute of every day. Figure out how you're using your time.
Now ask yourself, what are the things you absolutely cannot give up? For most adults, that will be your job or career. If you're in school, it will be your schoolwork. If you have obligations to family, that goes on this list too.
Depending on your age, lifestyle, and family commitments, this will vary widely. A forty-five year old lawyer who supports his wife and three kids has a much different set of commitments than a twenty-five year old single coffee barista who splits an apartment with four of his buddies.
What WILL you commit to this?
This is really the bigger question for most people getting into the martial arts, and it's one that is important to lay out early. People who get into the martial arts sometimes get very into the martial arts. It can become a lifestyle, or an obsession.
I read an interview years ago with a gentleman who was the head of a particular school of Japanese swordsmanship--I want to say Katori Shinto-Ryu, but I may have that wrong. In any case, he said something that stuck with me. I am paraphrasing here, but this is the gist
Everyone praises Miyamoto Musashi as Japan's greatest swordsman, but for all of his skill, what did he leave behind? A collection of writings, and a very small school. He had no family, no friends. My family is not as famous, but we have preserved our teachings within our family for 34 generations. My children and grandchildren will carry on that legacy.
Dan John makes a similar point in Intervention, where he points out that we have people who trading in their future chasing grandchildren in an effort to lose a few pounds.
I have seen people lose friendships and relationships in pursuit of their martial arts goals. If those are things you are okay with losing, then that's fine...but be sure you are okay with losing them.
On the flip side, there are those people who want to have it all, but don't want to commit anything. They want to be UFC fighters, but can't train Monday night because they'll miss their favorite TV show, Tuesday night is poker night, Wednesday is hump day and they're just too tired, Thursday is another TV night, and Friday is drinks with the boys after work.
If you aren't willing to give up anything you're doing right now, you probably don't want to join a martial arts school.
Take some time and figure out exactly what it is you can and will sacrifice to reach your goal. If you have a family, I suggest you involve them in the discussion as well.
There is, by the way, no right answer to this question. If you have Tuesday and Thursday nights free, and you are comfortable using those two nights to diligently work on getting better at a martial art, that's fine. Likewise, if you want to give up all human contact and friendship for ten years to become the best fighter the world has ever seen, that's okay too...but do it on purpose, not on accident.