Below image shows spacing issue while using sub report on main report.
Red rectangle - Tablix of Main Report.
Green rectangle - First Cell of Tablix on Main Report.
Blue rectangle - Second Cell of Tablix on Main Report.
Black rectangle - Sub Report Tablix
Can anyone please help me to solve this. I want to show the sub report as very next to first Textbox.
Without knowing what you've tried thus far, there are several issues:
Have you positioned them closely enough that the distance between the two items in the designer is set to between 0 and 5 pts?
If you have positioned them between 0 and 5 pts apart, then look for objects on the report where the top of the object is below the top of the subreport but above the top of the tablix. During rendering, objects cannot overlap, so if one object, even if it is seemingly unrelated to the tablix and subreport, were to expand vertically it would push down the tablix leaving the excess white space. It looks like the object in the upper-right-hand corner of your image may be doing this.
It doesn't look like this solution will work in this case, but to prevent one object from pushing other objects, you can group objects inside a rectangle. In that case, SSRS will move the Rectangle if needed, but will not disturb the spatial relationships of the contents of the rectangle.
If this doesn't help, you can tell me more about your specific situation via a comment and I'll do my best to help.
