How to use locks, for DB noivces?

I've never used the locking features of Postgres before, and now I
find that I need to use them.  I read through the instructions, but
I've never taken a database theory course so I don't understand all
the terms they used.

Here's what I need to do: The database keeps accounts.  If one process
is accessing an account, no other process should be able to access it
at the same time.  I need to do the following logical sequence of
things:

1. Lock the account

2. Check to see if the change to the account is permitted

3. Make the change

4. Unlock the account

I need this so that there isn't a race condition.  Ie, if there are
$10 in the account, and one backen says "withdraw $9" and the other
also says "withdraw $9" at the same time, I need to make sure they
they don't execute at the same time, which would result in a negative
balance.

I tried to do this with SELECT FOR UPDATE but I couldn't get that to
work.

Thanks