Things you should do when you become a committer
As described at ProjectRoles, we have 3 levels of involvement in the project, as in many ASF projects. This page describes stuff you'd want to do when you become elected a committer – in other words, when you're given write access to the SpamAssassin codebase.
(This page is still likely to be under construction.)
Set Your Password
Generally, you should have sent in your desired password in 'htpasswd' format with the account request (I think that's the current idea).
The alternative is that, when your account is created, the ASF infrastructure person will have sent you a password for cvs.apache.org. This allows you to log in using SSH to set your SVN password, as described [http://www.apache.org/dev/version-control.html#https-svn here]. Here's a quick summary of what to do:
ssh -l MYUSERNAME svn.apache.org Password: [THATPASSWORD] svnpasswd
And enter your desired SVN password twice to set it.
Writable SVN checkout
When a contributor becomes a committer, you should check out the svn repository using the HTTPS url instead of the HTTP one, so that you have read/write access to the repo. To check out from scratch:
svn checkout https://svn.apache.org/repos/asf/spamassassin/trunk
or, if you have an existing read-only SVN checkout, to switch an existing checkout to HTTPS:
cd /path/to/checkedout/tree svn switch https://svn.apache.org/repos/asf/spamassassin/trunk
Changes to the SVN tree can now be checked in directly using svn commit.
The first time a file is checked in using svn commit, the committer will be prompted for their username and password. This will then be cached for all further commits.
You should first commit a change adding themselves to the CREDITS file. If you want to do additional test commits, something like adding a single newline to the end of an inoffensive file in a subdirectory, or removing it again, would do the trick.
The first time you commit something your commit message needs to be moderated through, so don't be surprised if it doesn't show up for awhile.
EOL-Style
You should also ensure that their checkout has the svn:eol-style property set to native throughout, as (apparently) this policy cannot be set server-wide. Do this by editing the file ~/.subversion/config and adding these lines to the end:
enable-auto-props = yes *.pm = svn:eol-style=native *.pl = svn:eol-style=native *.PL = svn:eol-style=native *.in = svn:eol-style=native *.t = svn:eol-style=native *.c = svn:eol-style=native *.h = svn:eol-style=native *.xml = svn:eol-style=native *.html = svn:eol-style=native *.css = svn:eol-style=native *.bat = svn:eol-style=native
Misc
You should subscribe to the security list by sending a subscription request to security-subscribe at spamassassin.apache.org.
The PMC members should grant their Bugzilla account 'EditBugs' status. I'm not sure how this is done, but Theo knows 
More Details
Be sure to read [http://www.apache.org/foundation/how-it-works.html how-it-works.html] – there's lots of useful stuff about ASF processes there.