First I would recommend you use a custom macro to hold your notes so that if you change your mind on their presentation you can implement the change by just changing the definition of your macro.
So let's call your macro \booknote{pageno}{text}.
I'll give you a very simple solution for the sorting part.
If you are planning of taking only few notes it could make sense to have TeX sort them for you. Some more experienced user can give you advice on that.
Instead, if you are taking a lot of notes it would make sense, for efficiency, to defer the sorting process to an external tool. This is what BibTeX does for example.
Now, we do not want to write a custom script and we do not need to!
If you are on a Linux-ish OS you already have the program sort available at the console. It sorts the lines of a file in some order.
Now it is just a matter of organising your files:
the main file main.tex:
\documentclass{article}
\title{My notes for Bla}
\author{myself}
\newcommand{\booknote}[2]{\item[Pag.~#1] #2}
\begin{document}
\begin{description}
\input{notes.tex}
\end{description}
\end{document}
the file holding your notes notes.tex:
\booknote{45}{My note}
\booknote{925}{another note}
Now since notes.tex has your notes one for each line we can sort the file with
sort -g -k 1.11 -t} -o notes.tex notes.tex
-g sorts numerically
-k 1.11 uses the first field as sorting key, and the key itself starts at the 11th character
-t} says that } is our field separator
So sort will see a line like \booknote{45}{My note} find the } thus seeing three fields \booknote{45, {My note and ``. It will pick the first, skip 10 chars obtaining 45 and use that as the key to sort the notes.
The drawbacks are: you cannot hard-wrap the text of the notes, you should not insert whitespace before \booknote although empty lines are allowed.
Every time you change notes.tex remember to invoke sort before you compile main.tex.
Another option is using mynotes.tex to store the notes in the order you want and notes.tex as the target of sort so you can also edit your notes in the order you prefer.
836: Here shall be the note for page number 836..and then usesortandsedto sort and format the result so you'd get sorted lines of the form\item[836] Here shall be the note for page number 836..or whatever. Just add the top and bottom suggested in David Carlisle's answer and your notes would be ready to typeset. – cfr May 19 '14 at 22:18