Query optimization with X Y JOIN

Hey guys, how u been. This is quite a newbie question, but I need to ask it. I'm trying to wrap my mind around the syntax of join and why and when to use it. I understand the concept of making a query go faster by creating indexes, but it seems that when I want data from multiple tables that link together the query goes slow. The slow is typically due to expensive nested loops. The reason is, all my brain understands is:

select
tablea.data
tableb.data
tablec.data
from
tablea
tableb
tablec
where
tablea.pri_key = tableb.foreign_key AND
tableb.pri_key = tablec.foreign_key AND...

From what I read, it seems you can use inner/outer right/left join on (bla) but when I see syntax examples I see that sometimes tables are omitted from the 'from' section of the query and other times, no. Sometimes I see that the join commands are nested and others, no and sometimes I see joins syntax that only applies to one table. From what I understand join can be used to tell the database the fast way to murge table data together to get results by specifiying the table that has the primary keys and the table that has the foreign keys.

I've read all through the postgres docs on this command and I'm still left lost. Can someone please explain to me in simple language how to use these commands or provide me with a link. I need it to live right now. Thanx.