How can one become a Sotāpanna ?

My advise is that, first, one must strictly observe the 5 precepts (Panca Sila = Penta Sila) of Buddhism.

1) No killing of other living animals, from other human beings down to even tiny creatures like insects. (Plants and microbes can be killed, since they don't have minds.) No harm or pain is to be inflicted to other animals.
2) No stealing of other peple's belongings, valued 5 US Cents (1 Thai Baht) or more.
3) No sexual relation with other people's spouse or person under a protection of existing parents, brothers or sisters.
4) Speaking no lie. (This can be later extended to include bad-mouthing, sarcasm, and non-sense chatting.)
5) No alcohol, narcotics.

After accepting these rules, life of dharmma practitioner will gradually improved over a period of few months.

Second, one can also concurrently practice daily meditation. Samatha meditation is to let the mind rest at the serene peaceful state. Vipassana meditation, often done alternatively to samatha in the same sitting session, is to observe things and they happen, esp. one's own mental activity. Try to separate "the observer" from the emotional objects under observation. Then one day, one would gradually feel that the physical body and the conscious mind are separable entities. This is the starting process of dismantling of your selfness.

Third, to frequently use wisdom, in considering things as they are. I am talking about "the 3 characters" of everything (except Nirvana) here. That they are always changing (quick or slow, it does not matter, even the Everest only started to form just few million years ago), that after they start they always end (i.e. impermanence), and that you don't have any mental control over them. These are called the Tri-Laksana (i.e. the 3 characters)

After a long period of these 3 prong practice, the mind will soften its stance on "selfness", and accept the law of Tri-Laksana. One day the mind will accept that selfness is always changing, is impermanence, and is independent of mental control. This is the eureka moment of the mind attaining the Satapanna state.