I'm new to GIS. I want to calculate the elevation of any point on earth giving lat, Long (for example like google elevation Api, or the earth tools: http://www.earthtools.org/webservices.htm # cheigit) based the ASTER GDEM MODEL V2 I went to download data from there, and gives me geotif files. Then what should I do? Thanks
Each pixel/tile in the geotiff represents a height measurement. Since it's not an ordinary but a Geo-referenced tiff (raster image) you can compare it with any other georeferenced polygon or point location to recover the height measurement(s) that apply to it.
Before we help you any further I suggest you try and combine your set with the geotiff and show us what you're doing exactly. For more info on how to communicate this, see also this link