You Tube is one of the free sources to learn your desired course. Another option you can try is Swayam or NPTEL. It is an initiative by seven IITs and IISc for creating course and funded by MHRD. You just need to register yourself there. The courses are taught by professors of IITs and you will get weekly assignment which you have to submit online. Each course contains materials that can be covered in depth in 40 or more hours. After completion of your course you can give online exam where you need to score at least 40% score to get certificate.