For me, the first thing is thinking about my living situation, which you're already debating right now. Do you have space for a baby, and do you feel safe and stable there?
The next thing is your career. What job do you have? Do you want to keep it after the baby? Do you need to finish your education first? Is your job family-friendly?
Then, finances. Can you afford not to work or to pay for daycare? Do you know how much daycare costs in your town? Can you afford necessities like health insurance for the baby? Do you have family around who will help? Can you afford the prenatal care and childbirth costs?
Also, your relationship - do you feel stable and safe? Do you both feel ready to be parents?
Finally, your body - are you in good health? Are you getting good nutrition? It is best to get a preconception checkup.
I find these two articles helpful:
There is an AWESOME mental preparation for labor book called Birthing From Within that addresses the emotional aspect of becoming a mother. I'd def recommend that, but I guess it's a little more down the line. However, a lot of it can be extrapolated to TTC and just mentally/emotionally preparing for change no matter how far in advance.
My friend and I have a joke that we should give a strawberry plant to every woman who gets engaged because it takes a few years to really get good berries out of one. By the time it grows big, juicy fruit you'll be pregnant and craving them. Haha - so buy a strawberry plant to prepare?