In your Question you say you have downloaded the application (called checked-out, checkout)
From there, in your terminal you will need to cd into your project folder, then from there (where there will be .svn files) you can issue commands like:
svn status
to see all of your uncommited changes, changed files that you haven't told svn to send to your svn repository where the latest code is kept (can be local or remote server).
svn update
to get the latest code from the svn repository, this will be unnecessary on a one man project.
svn commit relative/path/to/file
this will tell svn that you have finished editing this file and to take a snapshot (called a revision) you can include many paths and files and i think you can use wild cards (i'm fuzzy between git and svn).
If you change some code in one file that some calls in other files will fail if not updated, all of these files should be in one commit if possible with a very clear message (you will thank yourself for clear log messages in the future, down the line)
and that is the basics of svn, sorry if i have merged (haa!) git and svn commands together.
After all of that i forgot to mention the rapidsvn package in the ubuntu repos, quite a good svn GUI in my opinion. Similar menu options as the commands i listed above with a very easy right click and revert option, check it out if your not a command line person.