If you are a GitHub user, you can use the Embed GitHub Gist plugin to do so. That will give you a nicely formatted code block with many options.
If you just want to use the WordPress editor and write your code, I've had good luck with SyntaxHighlighter Evolved and WP-Syntax. You just have to pick which one you like best. Oh, and at this very moment, I'm using the Twenty Twelve Theme. I'm looking at some other responsive ones (including Frank), but haven't pulled the trigger yet.
Are you posting in the right place? Check out Where do I post X? to know for sure.
Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
<h5> <h6> <hr /> <i>
<li> <nbsp> <ol> <p>
<small> <strike> <strong>
<sub> <sup> <table>
<td> <th> <tr> <tt>
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
Want more info? How to link
or How to display code and escape characters
are good places to start.