The path of a bodhisattva starts with taking refuge. Through understanding that samsara, the endless cycle of birth, life and death bears incredible suffering, one seeks protection in the Three Juwels in order to escape this cycle. On basis of refuge and by following the Four Noble Truths and the Noble Eightfold Path, the basis for final liberation from suffering is formed.
By understanding that not only oneself is subject to this cycle but all sentient beings suffer likewise, a starting feeling of loving kindness and hence compassion arises. Those respectively make the basis for entering the path of a bodhisattva. The path starts with the so-called bodhisattva vow which is being renewed, requested and received each life time from anew by the aspirant until all sentient beings will have attained enlightenment. Very much simplified it is the personal promise to finally reach enlightenment for the benefit of all sentient beings; to accord our actions and entire life with this aim and destination.
Having received this vow itself doesn't already make us a real bodhisattva, a being who at least abides on the first level of a bodhisattva (1st bhumi), more it creates the sound basis for our mind to be conditioned to others and their ultimate wellbeing, complete enlightenment.
The bodhisattva vow is the basis of Mahayana Buddhism with the strong foundation of Hinayana, a strong sense of renunciation of Samsara, with the practitioner living in accordance with the Four Noble Truths and the Noble Eightfold Path, practising the Six Paramitas in order to attain enlightenment. It is hence also one of the pre-requisites of Vajrayana.