indexing

Hi all,

I've been doing a little reading on indexing in prelude to indexing my db.
I have the following to ask:

if I had three tables for a many to many relationship say A, B, AND C
B being the lookup. B being a huge 50k rows plus column and made just two forigen keys(b.a_id,b.c_id).
is it best to create two non-unique indexes or one unique index on both fields?

Please advise

Ps

I also attempted creating an index on a table i have called person on person_id and
ran
Explain
select person_id from person where person_id < n

but saw no results of my created index being used - am i doing something incorrectly

Pps
When indexing if searching tables is more important than concurrency - which type of index is best?