i cant modify shape of text
If what you want to modify the shape of the text itself, you have to first convert the text to outlines, in other words you want to turn the text into objects. You go to Type>Create Outlines. Now you can manipulate the text anyway you want, each letter will have anchor points, you can distort each letter by itself. However, once you have done this, you CANNOT edit the text as text anymore.
If what you want to do is change the font, or the size, or whatever, make sure you have the correct text tool selected.
The text on a path tool is the T tilted over a line. You cannot use this on a path that you've turned into a compound path (merged shapes together, often done to cut out areas) or a masking path (an object that masks out something, that is, hides part of it, like you might use text to mask out part of an image so the image appears as if it's inside the letters).
Did you convert the text to outlines and now you want to manipulate the letters? If so, use the Selection arrows, the Direct Selection (hollow arrow) will allow you to select specific anchor points. You may have to ungroup (Command + Shift + G on the Mac, ctrl + shift +G on the PC) the letters (remember they are no longer TEXT, they are only shapes, so you cannot use the text tool with them.) Then click outside them and then click again with the direct selection arrow on the shape (letter) you want to modify.
Hope that helps