I offer this post as a quick way to access a number of resources that will provide a nicely-paced introduction to this powerful database programming language. Of course, it helps a lot to know SQL, too, so check out the Other Useful Links at the bottom of the post.
I wrote a series of "PL/SQL 101" articles for Oracle Magazine several years ago. Here's a convenient index to all those articles:
1. Building with Blocks - an overview of PL/SQL, followed by coverage of some fundamentals
2. Controlling the Flow of Execution - conditional statements and loops
3. Working with Strings
4. Working with Numbers
5. Working with Dates
6. Error Management
7. Working with Records
8. Working with Collections
9. Bulk Processing with BULK COLLECT and FORALL
10. The Data Dictionary: Make Views Work for You
11. Wrap Your Code in a Neat Package
12. Working with Cursors
I added several "PL/SQL 101" posts on this blog as well:
Nulls in PL/SQL
Declaring variables and constants
Writing conditional logic in PL/SQL
Other Useful Links
Oracle PL/SQL home page - latest news, links to lots of other resources
Oracle SQL home page - 'cause what's PL/SQL without SQL?
Database for Developers: Fundamentals - an Oracle Dev Gym class that introduces to fundamental relational database concepts
Database for Developers: Next Level - an Oracle Dev Gym class that makes sure you have a solid foundation in basic SQL operations
PL/SQL Tutorial from DatabaseStar - a free seven-part tutorial on basic PL/SQL concepts, packed with easy to follow examples.