What to do before correcting regrettable life choices? If someone has lived a regretful life and want to correct himself, what should he do before he begins his efforts?

If someone regrets the choices made in the past and intends to change, where should they begin?
In Buddhism, unwholesome action bears unpleasant fruit in this or a future life. But when and how intensely such fruit arises is mediated by what we do now and henceforth.  The most efficacious means for reducing such negative consequences are through developing our virtue, our mental capacities and our wisdom.  Below, I'll ground this response in the teachings of Bhante Henepola Gunaratana (known to many as "Bhante G," Abbot of the Bhavana Society, author of "Mindfulness in Plain English," etc., see, for instance, https://en.wikipedia.org/wiki/Henepola_Gunaratana) who anchors his advice in the oldest Buddhist texts and commentaries.

I recall during two different week-long meditation retreats with Bhante Gunaratana his being asked this same question, What to do when we've done regretful acts in the past?, by two different people.  One person was a innocent-looking young woman, the other a middle aged man who mentioned he had served time in prison and who appeared stooped over and brow-furrowed by deep remorse.  To each of these people, Bhante G gave the same response.  He first quoted from the "Lump of Salt Sutta" and then told the story of the mass-murder-turned-monk Angulimala.

In the Lump of Salt Sutta (in Pali, Lonaphala Sutta, AN 3.99 or 3.100, depending on which redaction; here's an English translation by Thanissaro Bhikkhu: http://www.accesstoinsight.org/tipitaka/an/an03/an03.099.than.html; and, here's a Pali redaction: https://suttacentral.net/pi/an3.100), the Buddha essentially asks his disciples, would a lump of salt have the same taste in a small bowl of water as it would in the river Ganges?  The disciples state that the lump of salt would make the bowl of water undrinkable but would not affect the taste of the Ganges.  The Buddha then reveals that the lump of salt is a metaphor for bad karma and that the water reflects ones spiritual development.  More specifically, bad kamma will have a harsher impact on one who is "undeveloped" (represented by the small bowl of water) while it will have a nominal impact on one who is "developed."  Developed in what way?  To quote from the sutta:


developed in body (bhavita-kaya)
developed in virtue (bhavita-sila)
developed in mind (bhavita-citta)
developed in wisdom (bhavita-panna)


(An earlier part of the sutta adds, "He is unlimited and has a lofty character, and he dwells without measure" [trans. Bhikkhu Bodhi], which some commentators associate with ones practicing lovingkindness, compassion, appreciative joy and equanimity, for instance, as indicated in the Bramha-vihrara Sutta, AN 10.208 [for which here is a translation by Thanissaro Bhikkhu: http://www.accesstoinsight.org/tipitaka/an/an10/an10.208.than.html; also see Piya Tan's article, "How loving kindness can limit bad karma" available at https://discourse.suttacentral.net/t/an3-100-lo-akapalla-how-loving-kindess-can-limit-bad-karma-by-piya-tan/1801].)

Just a quick side note, if I may, the notion of developing ones virtue, mind and wisdom finds resonance in the more general Buddhist framework known as the Threefold Training (see, for example, this Wikipedia article: https://en.wikipedia.org/wiki/Threefold_Training).  The notion of "developed in body" is often interpreted in terms of sense restraint (Pali, samvara; for a more detailed exposition, see, for instance, the Samvara Sutta, AN 4.14, an English translation of which is available at https://suttacentral.net/en/an4.14).

As for Bhante G's retelling of the story of Angulimala, a mass murderer whose very name refers to the necklace (mala) made of his victims' fingers (anguli), the most important canonical source is MN 86 (Thanissaro Bhikku's translation can be found at http://www.accesstoinsight.org/tipitaka/mn/mn.086.than.html).  Realizing the endless cycle of suffering he has created for himself and others, Angulimala disavows his murderous ways and becomes a follower of the Buddha.  (As Angulimala recites in this sutta and in Th. 16.8  [e.g., Norman's translation can be found at https://suttacentral.net/en/thag16.8 ]: "Swept away in a great flood, I went to Buddha as a refuge.")  Once, when going for almsrounds, Angulimala saw a woman in painful labor and was overwhelmed by how to respond.  The Buddha taught Angulimala the protective verse: "Sister, since I was born in the noble birth [that is, became a monk], I do not recall intentionally killing a living being. Through this may there be wellbeing for you, wellbeing for your fetus."  This exemplifies how Angulimala's new commitment to a harmless life enables him to both liberate himself and give comfort to others.

As an addendum to what I have learned from the wondrous Bhante G, I'd like to add two observations.  The first is that, among monks and nuns, there are communal processes for addressing interpersonal (and other) wrongs that include bimonthly communal confessions.  (See, for instance, https://en.wikipedia.org/wiki/Uposatha#Monastic_practice .)  I don't know if any such process is established in Buddhist lay communities and would, of course, welcome others sharing their knowledge about such.  

Secondly, our regret is often intermingled with our own natural, understandable desire to be forgiven.  In Buddhism though, seeking such forgiveness is not achieved through debasing ourselves or some form of ritualized self-flagellation.  Instead, this impulse for forgiveness reminds me of a chant generally found in post-canoncial Theravada liturgies, "Khama Yacana" (see, for instance, http://chantpali.org/closing.html#khama_yacana), where the verses state in part:  "If by deeds, speech or thoughts heedlessly I have committed any wrong-doing, forgive me, O Bhante ... O Dhamma ... O Sangha ...."  In such a way, beyond the desire to resolve our karmic consequences and our desire to create communal harmony, we can try to temporarily ease within ourselves any excessive burden, facilitating the realization of liberation.