How A/B Testing and Multivariate testing going to be done dynamically?

How A/B Testing and Multivariate testing going to be done dynamically?

HI, 
I am new to the A/B testing please guide me how to perform

How A/B Testing and Multivariate testing going to be done dynamically?
1)How variants are going to be generated
2)How visitors are tracked for each variant 

thank you.