If you want to compare two revisions in a file's history, for example revisions 100 and 200 of the same file, just use TortoiseSVN → Show Log to list the revision history for that file. Pick the two revisions you want to compare then use Context Menu → Compare Revisions.

If you want to compare the same file in two different trees, for example the trunk and a branch, you can use the repository browser to open up both trees, select the file in both places, then use Context Menu → Compare Revisions.

If you want to compare two trees to see what has changed, for example the trunk and a tagged release, you can use TortoiseSVN → Revision Graph Select the two nodes to compare, then use Context Menu → Compare HEAD Revisions. This will show a list of changed files, and you can then select individual files to view the changes in detail. Alternatively use Context Menu → Unified Diff of HEAD Revisions to see a summary of all differences, with minimal context.