To get an expression exp to appear as a subscript, you just type _{exp}. To get exp to appear as a superscript, you type ^{exp}. LaTeX handles super-scripted superscripts and all of that stuff in the natural way. It even does the right thing when something has both a subscript and a superscript. You must provide formula or equation environment to our expression and this is provide by using $ exp $. For example

i) Writing subscript :

$ x_0 $
$ l_{eq} $
$ v_{(x,y)} $

ii) Writing superscript :

$ x^0 $
$ l^{eq} $
$ v_{(x,y)} $

iii) Writing both subscript and superscript :

$ x_0^0 $
$ l_{eq}^{eq} $
$ v_{(x,y)}^{(x,y)} $

Here _ is for subscript, ^ is for superscript and {} is used to treat every thing it it as a single unit.