I want to know; How can I emphasize the sentence? I mean, when I write for example: Can I...
And if I use: IOS, then what/how can I do in order to do this?
Or, If I want to emphasize a whole text so that it will be 100% clear. How can I do this?
You can't change the color of text, see the thread on meta. You can make something bold or italic for emphasis.
To do this, do either one of:
The more important thing to work on is to keep your text concise, coherent and neat. If you absolutely need to write more than a few sentences then put them in proper paragraphs. People don't like to read long chunks of un-punctuated rambling.