Well, that's an awfully general question. Out of factors that you have some influence over (as opposed to e.g. your own IQ or temperament):

First, good genes. Yes, you can't change yours but you have a choice of a set out of which the second half will be picked. Use it wisely :-)

Second, money in its usual function of being able to get you what you need including such things as e.g. safety and, to a limited extent, time. Of course there are the usual problems in that money itself is commonly a trade-off against your own time and stress, so this is highly dependent on the circumstances. But children are expensive :-/

Third, a support network of relatives and friends (hopefully including your spouse).

Fourth, later, a good street/school environment.

Of course, people managed to bring up kids lacking all of the above, so it's not a set of necessary things, only useful ones.