SQL Server 2005 Microsoft put out a Dashboard report that is very useful for getting a good overview of what resource usage is, you can download that here. This is a quick post about using it with SQL Server 2008. That I know of this is reports that you run on the fly, I don't know if you can schedule them but I'm sure someone has.
I also believe with SQL Server 2005 if you are at SP2 or higher there are some canned reports that will show up in SSMS for you. Then with SQL Server 2008 the activity monitor is pretty good place to start, there are some good improvements in it compared to SQL 2005.
You can do a search on the web for checklist and such, or "how to monitor __" that will get you started. Some things are going to require you to capture the info from system views in some manner over time so you can pick up the trend (example: database size growth). It is just a matter of finding what you find the easiest to implement and gives you what you want without effecting performance on your systems.