These commands are used to deal with tagging and branching in SVN:

$ cd /path/to/project && svn copy trunk branches/mybranch-N && svn commit -m 'Created new branch'
$ cd /path/to/project/branches/mybranch-N

Make changes and commit:

$ cd /path/to/project/trunk
$ svn merge -r R:head svn://path/to/repo/branches/mybranch-N

(In the merge command, 'R' is the revision in which the branch was created. Note that if the trunk is merged into the branch, then merging the branch back into the trunk again will cause conflicts if selective merging is not used. Use the svn merge -r start:stop syntax for selective merging.)