First, you can connect WIFI (or) modem with the internet in into your computer and open any type browser and go to google and search for YOUTUBE.com on that browser.
Now page is opened and search the video which videos what do you want you can search any type of videos,music,movies.Search your video and that type of video will show you list of videos and play video what do you want to download.
Open the video and paly the video for few seconds and you can't download any type of software to download that video you can download it directly into the browser.
Now type double ''SS" just at the beginning of the Y of YOUTUBE into "URL" Box and press enter and you will be directed to a new page to press enter.You will have different download option available there it will direct to a new page which will have all the options for download it open Savefrom.net.The resolution are given there like FLV240,MP4 360P,MP4 720P,3GP 144P,3GP 240P and audio MP4 128.
Select the format in which you want to download click on the desired format and you can select the format you want to download in format select "MP4" and click on that type of format.Your IDM will pickup the link and start downloading if you don't have an id,then your browser will start downloading video itself.In this way, you can easily Download YOUTUBE Videos.