How blood cancer will occur?

Are you asking about symptoms or causes?
As yet there are very few risk factors for either leukemia or lymphoma - where lymphoma is concerned sometimes the Epstein Barr virus can be a factor as can working in a pulp processing factory but it is not like smoking in that it is always a direct result or massive risk factor.
As for symptoms - for both types of blood cancer one of the major symptoms is fatigue especially if it is a major change. Add into weight loss without dieting and becoming easily bruised (and I mean huge bruises from just someone holing your hand), large lumps in your neck, shortness of breath, unbearable itching and night sweats then it's time to see the doctor.