I just downloaded this app on my android phone. I don't know how to post mathematics problems here. Can anyone suggest me how to do that? In this app there isn't any help for that problem how to post.
The Android app is far less developed than the site itself. It can be convenient when you already know how to use the site (especially now that the app has enabled MathJaX), but there's hardly any guidance regarding what the site is. The target audience for the app, at least at this stage, consists of established Stack Exchange users.
For writing formulas, see MathJax basic tutorial and quick reference but be warned: typing on a smartphone or tablet can be very annoying, because you'll be constantly switching keyboard panels looking for
\, and so on. And if you are also using LaTeX syntax for the first time, the experience will be nothing short of torture.
I can recommend the app for browsing (although filtering options are scarce compared to the site), checking the inbox and commenting, but not for posting mathematical questions or answers.
The default keyboard is only adequate for typing alphanumerics. For the rest of ASCII, anything in an extended Latin character set, or a functioning CLI, one needs a replacement, such as Hacker's Keyboard. A benefit of a good soft keyboard is the ability to type in multiple languages, in Unicode — this keyboard has, for example, ∞ attached to the 8 key and π attached to p, along with layouts for Ελληνικά and пхонетиц Русский (or the correct version).