In order to checkout a repository do the following.
Create a new folder somewhere and open it.
Right click it and select SVN Checkout...
In the new window type in the URL for the repository. I suggest you use the following URL, this way you only get the main development line:
https://clamwin.svn.sourceforge.net/svnroot/clamwin/trunkMake sure the checkout directory is correct.
Press ok.
TortoiseSVN should now start downloading the repository, this might take a while.
I suggest you have a look at the TortoiseSVN manual in order to get familiar with TortoiseSVN's abilities and SubVersion repositories in general.
http://tortoisesvn.net/support