<p>I would like to know if there are general rules for creating an index or not.
How do I choose which fields I should include in this index or when not to include them?</p>

<p>I know its always depends on the environment and the amount of data, but I was wondering if we could make some globally accepted rules about making indexes in Oracle.</p>

There are some things you should always index:
Primary Keys - these are given an index automatically (unless you specify a suitable existing index for Oracle to use)
Unique Keys - these are given an index automatically (ditto)
Foreign Keys - these are
not
automatically indexed, but you should add one to avoid performance issues when the constraints are checked
After that, look for other columns that are
frequently
used to filter queries: a typical example is people's surnames.