Im a beginner to video editing, kindly help me to select (free select, as like in Adobe Photoshop) particular portion of a video, to merge with another video?
To build on AJ's answer in a different direction, you may want to look at After Effects, which is a popular compositing tool. In After Effects, you can use the pen tool (somewhat like the lasso tool) to create a custom shape. With that shape, you can create a mask for the video. This mask can then show only the video inside of the shape.
Further, the mask can change over time with your video. If the subject of your first video moves, you can adjust the mask's shape as well. In other words, the mask can be animated, though this process can take a lot of time.
If the element you need to put on the video doesn't move and the shot you are putting it in doesn't move, the best bet is to actually use Photoshop to extract the object you want to place in another video and put it in a file format that supports a transparent background (TGA, PSD, etc.) You can then import it to Premiere and place the image wherever you want in the frame using the Motion effects settings to define the size and placement.
If the video you are placing it in is not stationary, then it will be necessary to perform motion tracking on the video you want to place the image within. This is much more complicated and is generally done in something like After Effects.
Unless I'm misunderstanding your question, I would try a
Garbage Matte effect (either 4, 8, or 16-point). Stack the two video clips on the timeline (e.g. one video clip on track V1 and the other on V2). Drag n Drop the
Garbage Matte effect (8-point is a good starting point) on to the top video clip. Activate the
Garbage Matte effect in the Effect Controls window and then drag the points to select the portion of the top clip video that you want.
Only the part of the top clips video that lies within the bounds of garbage matte markers will show through. The rest of the video will be the bottom video clip.
This is reasonable video on the subject. He basically merges two video clips of the same scene with different exposure levels to get a single video with good exposure throughout:
Search youtube for Garbage Matte premiere and you will find several helpful videos on it.