As a novice user how can I reinstall an old Mac?
Technically this isn't an answer, but it's way to long for a comment.  If you come back with more details I will tailor the answer to suit your circumstances.



Where can you find out?  Asking here is a great start :)  I'll try to put some pointers your way below, but if you have the time and money, then I highly recommend the "Missing Manual" books (a different one exists for each version of the OS) which are probably the most comprehensive and accessible of any Mac user guides I have seen and read.

It would be really useful to know exactly what model the machine is, and what the current OS level is to understand any possible upgrade options etc.

Assuming you can log in and use it in any capacity, then click the apple logo in the top left of your screen, directly under that click About This Mac, you should get a box like this:



This gives us basic information about the machine, it's CPU and RAM (which will let us understand what OS is is capable of running) and it's current OS Level.

The best way to prep the machine is to wipe it clean and install the OS from scratch

If you got any install DVDs with the machine (they will have been supplied but whether they have made it to you 5 years later is another matter) you can factory reset the device to it's original state with those, but there is every chance that whatever it is currently running is newer than what it was supplied with originally.

If you don't have the discs, then you need to get some.  However it's not quite that simple, as these days all OS installs and upgrades are downloaded from the internet.  If your current OS is not 10.6.8 or higher, then you don't have access to the Mac App Store which is where you can purchase the upgrade from.  If it's 10.6.&lt;8 that you can at least upgrade the machine to 10.6.8 by simply running software update (Apple logo &gt; Software Update).  If you are on 1.05.x or earlier, this process is harder, but it can be done.

Assuming your machine will support it I would recommend running the latest available OS.  It's not expensive ($30 or so?) and will provide the best experience f you are new to Macs.  Now, there's every chance that either you don't want to for some reason, or are already on the most recent version (as per my screenshot) etc.  If you are on 10.7 or above you can try a re-install using the recovery partition which should allow you to wipe everything clean and start again.  No point adding instructions until we know what you are on or what you want to do, but we can clarify later how to do this.

Lastly, without touching the OS, there is a quick way to at least get the thing running with default settings on the current version you have which is to create a new user account, and log into that instead of whatever you are using now.  Click Apple &gt; System Preferences... &gt; Users and Groups and add a new user:



When you login as the new user all the settings and such will be as if you had installed the OS from scratch, but there may be non standard applications still available etc if they existed before, and other users files and settings will still be taking up disk space etc.