Hello aq,

Duration of the course depends on the institution, but there's minimum of 3 months course I'm digital marketing, which can also give you a good knowledge and opportunities to excel in this field.

You can checkout websites like upgrad and others, they give high quality education and best courses from top tier teachers.

It's better if you pursue your education after 10th also, and you can enroll in the course side by side,

You need to clear the course, and you can combine it with different skillsets to get the job/start your freelancing services for the same.

Mostly digital marketing is best combined with SEO/web development/graphic designing combined.

I hope it helps.

Best of luck