can we work in another company other than the company which sponsored me h1b?

Hi, right now i have 2 yrs of work experience and i want to apply for h1b visa, i have few doubts regarding this?

1.One of my relatives have a s/w company their in us, if i can convince them to sponsor me h1b and after landing in us, can i try for some other job and work in another company other than the employer who sponsored me as i cannot request for a job, if yes then is it legal to work like that??

2.Also for how many yrs i will get the visa?

3.There are lot of consultancies in US, with the help of consultancy can we get a h1b?