Paras
This is a very broad subject. Do you want to know about AC/DC, DC/DC, isolated? Depending what you want there are also many side issues like safety and EMI.
There are excellent books on the topic, but also semiconductor manufacturers provide application notes, on-demand courses etc. as well as design tools. I would look at such manufacturers as Microchip, Texas Instruments (and especially their webbench design tool if you want DC/DC), Maxim, Analog Devices/Linear Technologies, Power Integrations to name but a few.
