Adding of indexes, best practises

Good day all

Maybe someone could help me out from a resource point of
view or even from an experience point of view.

I have been a dba for only a short while now and the
training I did was not as intensive as you would get in cases like Oracle
for instance.

I am currently involved in a project for one of the clients
where they are busy consolidating and migrating some of their systems.

One of the tasks I need to do is to look at some of the
queries that are quite lengthy and running for long periods, and then
determine whether there are any indexes that should be added, etc...

I know I can use the explain option to see what the query
time will be as well as some other info, however from experience point of
view this does not mean much to me yet.

Can someone direct me to some info or assist regarding the
following:

. What does the explain output mean and how to interpret the output
to determine how queries can be optimised.

. Is there a guideline in terms of when / where indexes should be
added or will be beneficial? How do I determine of the query needs and index
or not.

I would really appreciate the help as I am still in a learning phase.

Regards

Machiel