How do I remove "Niraj", (The person on every sketch at start up) from showing up every time I open a new sketch?

Make your own template without him. Start a new SketchUp session, delete him and then in Window>Model Info>Statistics click Purge Unused. Then use File>Save as template to save it as your own template.

While you’re at it you might want to make changes to the style. I prefer a white background, Profiles set to 1, and use a green color for back faces. Also turn off Length Snapping in Model Info>Units and choose the appropriate units for what you’re modeling.