How to write a portion of text on the right only?

You can use hagindent and hangafter TeX primitives after the text is measured using setbox primitive:

defp #1: {par setbox0=hbox{#1: }hangafter=1hangindent=wd0 noindentbox0 }

p Sometext here: bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla
                  bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla
                  bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla
                  bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla.

The following approach uses paracol to set a two-column environment with the left column containing only the heading.

documentclass{article}

usepackage{paracol,lipsum}

newlength{sometextheading}
newenvironment{sometext}[1]{%
  settowidth{sometextheading}{#1:}%
  setcolumnwidth{sometextheading}%
  par
  begin{paracol}{2}
    noindent #1:
    switchcolumn[1]noindentignorespaces
}{%
  end{paracol}
}

begin{document}

sloppy% Just for this example
Here is some text

begin{sometext}{Some text}
  lipsum[1]
end{sometext}

begin{sometext}{Some more text here}
  lipsum[2]
end{sometext}

begin{sometext}{Yet more text}
  lipsum[3-6]
end{sometext}

begin{sometext}{Some closing text}
  lipsum[7]
end{sometext}

end{document}

You can do that with a description environment, customised with enumitem. The interesting point is that the contents of an item can break across pages.

documentclass{book}
usepackage{enumitem}
usepackage{calc} 

begin{document}

begin{description}[font=normalfont, leftmargin=widthof {Some text here: } + labelsep]
  item[Some text here: ] Blah blah blah blah …… blah blah blah blah blah blah blah blah. 
  Blah blah blah blah …… blah.
end{description}
begin{description}[font=normalfont, leftmargin=widthof {Some other text here: }+labelsep]
  item[Some other text here: ] Blah blah blah blah …… blah blah blah blah blah blah blah blah. 
  Blah blah blah blah …… blah.
end{description}

end{document} 

Here's a very simple solution: use itemize with your left hand text as your custom 'bullet symbol'. This doesn't require any packages, or much code, and it makes all right hand text aligned.

documentclass{article}
begin{document}

begin{itemize}
 item[Some text here:] Lorem ipsum dolor sit amet, consectetuer adipiscing elit.  Donec hendrerit tempor tellus.  Donec pretium posuere tellus.  Proin quam nisl, tincidunt et, mattis eget, convallis nec, purus.  Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus.  Nulla posuere.  vspace{2ex}
 item[Some other text here:] Pellentesque dapibus suscipit ligula.  Donec posuere augue in quam.  Etiam vel tortor sodales tellus ultricies commodo. Suspendisse potenti.  Aenean in sem ac leo mollis blandit.  Donec neque quam, dignissim in, mollis nec, sagittis eu, wisi.  Phasellus lacus. 
end{itemize}
 
end{document}