How to use web data from GSM Sim900 to be used as input for Arduino?
You should read the manual.
More specifically the "Sim900 AT reference".
I've found it on a russian server (https://www.google.nl/url?sa=t&amp;source=web&amp;rct=j&amp;url=http://wiki.amperka.ru/_media/%25D0%25BF%25D1%2580%25D0%25BE%25D0%25B4%25D1%2583%25D0%25BA%25D1%2582%25D1%258B:gprs_shield:gprs-shield_sim900_at_commands.pdf&amp;ved=0ahUKEwinze2Tp7fQAhWBMSYKHQFkBh0QFghEMA0&amp;usg=AFQjCNGKd5Uohe7CmcJcIc1Am-SKdRsWXg&amp;sig2=XIsvvkLWHs60sTo7FZ7tWw) but you may want to try and get it directly from the manufacturer.

In the datasheet, it will explained what values are allowed with the "at+httpread" command. You can actually specify which part you want to read. This is usefull when your Arduino can't parse the complete page.

If you have control over the page, you may also generate a device-friendly page consisting of key-value pairs, instead of it being a whole HTML page.