How to Force Transactions to Process Serially on A Table

I am using PL/SQL functions on Windows 2003, version 8.0.1.

I have not used explicit PostgreSQL LOCKing before, but I need some advice
on how properly to use some explicit locking. I think that duplicate key
violations I am now getting are the result.

I want to force transactions being used to update a table to be processed on
a first-come, first-served basis. I want my Pl/sql function to execute to
completion on each transaction before another starts.

Need some advice on how to do this. From what I can read in the docs, it
looks like I need to solve the problem by using the following, but doing so
gives me an SPI_execution error: BEGIN; LOCK <table> IN SHARE ROW  EXCLUSIVE MODE;   lock adns_report_hour_history in
sharerow exclusive mode; INSERT INTO <table> VALUES ...  - or - UPDATE <table> SET .... COMMIT;
 
Will this make the next transaction wait until the previous transaction has
completed? Do I need to set any config parameters?

If you can include an actual code snippet in the response, it would help ...