Macs do not generally require defragging. They have a number of safeguards built in which prevent it, although sometimes you will need to defrag it if you try and install Windows and it can't find enough clear space. It will tell you if you need to do this.The easiest way to defrag a Mac drive if you ever have to is to reformat the drive, reinstall OS X and restore from Time Machine (though it sounds complicated, it's not really). This explains it: http://macs.about.com/od/faq1/f/defrag.htm
There are a number of other things you can do to clearn your Mac, though. My personal favourite is the application OnyX. It can runs all the maintenance scripts and clear all the caches you need to, amongst other things. Head to the Automation pane and select all the boxes, then click run (it could take a while or not very long at all). This is where you can find it: http://www.macupdate.com/app/mac/11582/onyx
You might also find it useful to remove as many desktop icons and files, menu bar items, dock items, start up items and Dashboard widgets as you can (especially the widgets). Try also removing some of the junk you may have accumulated, like apps and media. A good way to delete apps properly is to get the app AppCleaner. It deletes not only the app (which is simple enough) but the related files as well (a little harder to do and often forgotten). I use AppCleaner and find it very effective. http://www.macupdate.com/app/mac/25276/appcleaner
Reset Safari. Open it up, click its name in the menu bar and click Reset. It'll clear out saved passwords though. Also try and let go of some of the plugins you may have installed because more often than not, they are just bogging the browser down.
Another handy app is Monolingual. It can delete excess langauge packs you don't need and save some space on the drive. I wouldn't bother if you have lots of space on the drive left though, because there isn't that much gain unless you're desparate for space.
Lastly, a good old fashioned 'turn it off and on again' every week or two can do wonders for you Mac (although a lot of the benefits can be achieved through OnyX).
Hope your machine speeds up a bit
How do I clean my Mac? On a PC I would go through and defrag my drives every now and then. Do I need to ...
Whatever you do, never download or install any product claiming to magically "clean up" or "speed up" or "optimize" your Mac. Without exception, they will do the opposite. If you already did, you found the problem. Fix it.
Macs have never required defragmenting: http://support.apple.com/kb/HT1375
Macs do not need "cleaning". Do not use non-Apple tools to delete programs or files. Most of them will cause problems that will only result in you coming back to this site with complaints. If you bought or installed a program that included its own uninstaller, use it. For everything else, use the Finder to delete unwanted files.
A reliable program that claims the intelligence to automatically discriminate between required and superfluous files does not exist. Such one-size-fits-all "clean up" utilities frequently don't know the difference between required system components and trash, and can easily result in a broken Mac.
Monolingual in particular can delete essential system components and has been known to break app updates:
Cache "cleaners" such as Onyx have been unnecessary for years. OS X performs its own administrative tasks so you don't have to.
Macs are designed to used, not pampered, and maintain themselves very well without all the attention you may have been accustomed to having used PCs. Do yourself a favor and drop that mentality. You will save yourself time and frustration.
General Mac maintenance: Tips to keep your Mac in top form <--- this is all you need. Really.