Indexing To Prevent Timeout Expired Message

sorry for posting a question that has probably been posted hundreds of
times, but i can't seem to find the answer i need...We're using a Sql
Server 7.0 database and recently started getting timeout expired
errors on our views, which i'm assuming is due to the volume of
records in our tables. Unfortunately i'm not too experiences w/
indexing tables, so my question is:
1) how do you determine which fields to index (does it have to be a
field w/ no nulls, no duplicates..similar to primary key), and why not
index the primary key
2) is it better to index the same field on different tables, or is it
a better design to index a different field for each table (ex. do you
want to index the same field that you link the different tables with,
or do you want to avoid doing this)
3) can somebody explain the concept of creating a clustered index
4) once table is indexed, if there are users accessing the linked
tables via Access, do they have to re-link the tables?

sorry for the number of questions and what may appear to be simple
questions to most, but again, i'm pretty unfamiliar w/ indexing and
would really appreciate any help...thanks