If one were to accept upon oneself a single mitzva to perform as carefully as possible as a first incremental step toward full t'shuva, should that mitzva be a negative commandment or a positive commandment?
I imagine that answers to this question could come in anecdotal or legal form.
I agree with Isaac's sentiments; it depends on so many things.
But here's some food for thought:
Psalms (34:15) says "avoid evil and do good." Many thinkers (including I believe R' Henoch Lebovits ztl) say that they go hand-in-hand, part of how you avoid evil is by doing good. Often the correction recommended for violating (and further avoiding) a "don't-do" mitzva is by getting more involved in "yes-do" mitzvas. It can also build a person up and keep them from feeling hopeless or depressed if they slip up. For a contemporary example, it may be easier to keep a commitment like "I'll brush my teeth after meals" rather than "I won't eat junk food." Again, you have to know your own situation.
At the same time, some schools of thought have it that no, first you have to get rid of all the bad, and only then do good. Rambam rules that if someone is a big sinner, you don't teach them Torah until they've taken care of their problem. But I don't think that means that we all have to be perfect before we can ever set foot in yeshiva. (I think you could argue from there that a "yeshiva" shouldn't take kids with a drug problem, but you could certainly have a religious rehabilitation program that included Torah study.)
At the far end of this logic is R' Yoizel Horwitz, founder of the Novardok school of extreme mussar. R' Yoizel spent a great deal of time in seclusion, and missed going to shul one Rosh HaShanah; depending on the version of the story, he either blew his own, or missed shofar altogether that year. He explained that first he had to work on himself and get out all the bad, before he could start doing good. This raised some eyebrows in the rabbinic world.
There's a fascinating Meshech Chochmah I heard from Rabbi Lapin. (I think this is his comment on why the first day of Pesach (vis-a-vis Sefiras HaOmer) is called "Shabbos.") He says that plenty of peoples distinguish themselves by interesting rituals they do; we distinguish ourselves by what we don't do.
Generally speaking, the punishment listed for actively violating a "don't-do" mitzva is greater than that of passively not doing a "yes-do" mitzva (e.g. only the former can warrant lashes). So if someone is debating between giving up pork and putting on tefilin, that would be an argument towards first giving up the pork.
Famous story -- in the 1500s an imprisoned Jew explained to his warden that he needed to get out of jail so he could observe his religion; the warden offered him one day out of jail to get his religious fix, but then he'd go right back to jail. Which day should he pick? The Radbaz answered:
The first day you can get.
Many take this philosophically, take the first mitzva you can get. Some say it was pragmatic, who knows if the warden will change his mind tomorrow. But in our case, both apply: better to tell yourself "I'll start keeping this small thing today" than "I'll keep that big thing next week", as who knows how you'll feel next week.
To sum it up:
Pick something meaningful, effective, and achievable. And start now.
There is a story (I don't recall the details or the source) where a habitual sinner - if I recall correctly, a thief - asked his Rabbi for where to start in doing teshuvah. The Rabbi tells him to accept on himself to always tell the truth (which is actually abstaining from violating the negative commandment לא תשקרו, not to lie). This ended up working, because the next time the guy was tempted to steal, he realized that he wouldn't be able to lie his way out of it.
Another story has a Jewish soldier in WWI - pretty well assimilated, but having a warm relationship with his community's Rabbi - being advised to keep the mitzvah (again, a negative commandment) of abstaining from non-kosher wine, with the idea that this would help him maintain some concept of the differentiation between Jews and non-Jews. (The story goes on to tell how his careful observance of this saved his life.)
So there is a strong case to be made for starting with commitment to a negative commandment first.
On the other hand, the Chinuch (which I quoted in an answer to a different question, here) calls the mitzvah of tefillin, a positive commandment, "a great fundamental principle and a major protection against sin, and a strong ladder by which to ascend and enter into the service of the Creator." Historically, the Semag (R' Moshe of Coucy) indeed devoted a great deal of effort to getting religiously-lax Jews to start putting on tefillin (with this idea in mind), and in our generation the Lubavitcher Rebbe zt"ll did the same.
All of which, then, goes to support Isaac's and Shalom's points: there is probably no one-size-fits-all answer.
just read todday that Rabbi Aharan Shteinman was asked this and answered: "to strengthen in emuna and bitachon" as the verse "tzadik b'emunato yichye"
There is a Posuk in Tehilim Sur Mairo V'Asei Tov. I guess first you have to leave the bad and then do good.
