Subversion is great and has many features, but sometimes (specially when merges can be complicated) you might want to switch to working as you did in Visual Sourcesafe where you were the only one that could modify certain file until you check out.
A very common question I get is "I just want to work with subversion as I did with Sourcesafe!! How can I do it?"
One way to do that is to use SVN Locking.
Tortoise provides a great way to do that: see this page.
Figure 1: This is the dialog you get in tortoise with you right click a file and select GetLock
Figure 2: To check who has a lock on a file you right click and select Check for modifications