Its not that hard. The guide on the link provided is pretty comprehensive if you actually read it in detail. Alot of it is copy and paste into a command line and hit enter.
Re read the guide and break it down into stages. Lots of people are quite worried about this sort of thing first time but it could help you get to know abit more about the phone you have purchased and how to solve problems like this in the future.
The guys and gals on this forum are a wealth of knowledge and reading through a few wikis and how tos is a great way of gaining the skills and experience to get the most out the N900.
Good luck and be sure to know what exactly your installing in the future. Most applications have an ANNOUNCE thread where users with a bit more experience have tried the app out and reported any bugs