<p>As a follow up to "<a href="https://stackoverflow.com/questions/105400/what-are-indexes-and-how-can-i-use-them-to-optimize-queries-in-my-database">What are indexes and how can I use them to optimise queries in my database?</a>" where I am attempting to learn about indexes, what columns are good index candidates? Specifically for an MS SQL database?</p>

<p>After some googling, everything I have read suggests that columns that are generally increasing and unique make a good index (things like MySQL's auto_increment), I understand this, but I am using MS SQL and I am using GUIDs for primary keys, so it seems that indexes would not benefit GUID columns...</p>

In general (I don't use mssql so can't comment specifically), primary keys make good indexes. They are unique and must have a value specified. (Also, primary keys make such good indexes that they normally have an index created automatically.)
An index is effectively a copy of the column which has been sorted to allow binary search (which is much faster than linear search). Database systems may use various tricks to speed up search even more, particularly if the data is more complex than a simple number.
My suggestion would be to not use any indexes initially and profile your queries. If a particular query (such as searching for people by surname, for example) is run very often, try creating an index over the relevate attributes and profile again. If there is a noticeable speed-up on queries and a negligible slow-down on insertions and updates, keep the index.
(Apologies if I'm repeating stuff mentioned in your other question, I hadn't come across it previously.)