1. Remove the dust accumulated in your computer. It traps heat and prevents air circulation. And it is very important to keep things cool in your computer. You can use an air spraycan (sold in most computer component stores) or just blow it off (close your eyes when you do it). It's not as complicated as it sounds. In the worst case, you'll need to unscrew some screws to open the case, but some more advanced cases have fine nets that trap the dust, which can be easily removed without unscrewing anything (like my case). I recommend doing this once a month but it can vary from one PC to another.
2. Clean your CDs. If you play games from a CD (which is rather uncommon nowadays) or actively use CDs/DVDs/BluRay for burning/movies/etc, you might get corrupted files and your movies might skip if your CD is dirty. The best way is to clean with tissue paper and alcohol by rubbing OUTWARDS (radially) from the center to the edges of the CD. This way, there is less chance of damaging the CD. If you're planning to buy a CD cleaner device, do your research as some of them are less effective than others. Clean your CDs when they are noticeably dirty or have fingerprints on them.
3. Upgrade your computer components. Now this is a little advanced and might require some reading and research, but there are other alternatives if you're scared of touching anything inside your computer. First of all, if you have an old computer (pentium III and older) it is not really worth upgrading it since it's equivalent to buying a new engine for your 1990 Honda Civic, or in other words not worth it. The best and probably easiest upgrade you can do is upgrade your RAM. It can cost you easily under 80 dollars and allow you to run more programs at the same time before your computer slows down considerably. It can also be as simple as opening your computer and plugging the RAM stick into the appropriate place. However, you have to consider compatibility so don't just go and buy the first RAM stick you see! Consult a professional or do some research first.
This is what a RAM stick looks like.Software
4. Defragment your hard drives. This operation "organizes" the data in your hard drives and makes it easier to access files. This will generally improve responsiveness and reduce the time it takes to open certain programs and access certain files. The simplest way is to use the defragmenter included with your operating system, which does almost the same thing as any other software. I recommend doing it once in two months, since it's a lengthy operation. WARNING: if you have a solid state drive, DO NOT defragment it since it shortens its life span. If you don't know what a solid state drive is, you probably don't have one.
5. CCleaner is a nifty free software that cleans your registry and does a good general "housekeeping" in your computer. Google it up and download it. To use it, all you need to do is right click on your garbage bin and choose "run CCleaner" and it will do everything without any other intervention from your part. The best registry tool for lazy people! It works in the background and doesn't take many computer resources. It can also free up a considerable amount of disk space so use it often, preferably at least once a week.
6. GameBooster is another free program, but it's designed to "give priority" to a video game and shut down unnecessary processes temporarily. It's very useful if you have an older computer and the new version is very user-friendly. Activate it every time you want to play a graphic-intensive game but don't leave it on at all times, since you won't be able to use basic services such as printing!
7. Keep your computer clean from Malware and Viruses. My personal combination is Microsoft Security Essentials (for viruses) alongside the basic windows firewall (against hackers) since it's a good balance of protection, ease of use and computer resources. I also periodically use Spybot, a software that's been around for centuries and that's really effective at removing malware. It's also free so definitely worth a shot. Lastly, I discourage the use of P2P programs such as Limewire or Kazaa (not sure it still exists) as those are filled with viruses and malware and the files that seem safe are often deceiving! In general, be smart and careful with internet and think before clicking anything.
8. Disable and shut down unnecessary processes. Learn to check your traybar! Yes, the one that's in the bottom right of your screen. Look at every little icon and decide if you really need that application to be running or not. Applications that you need to use sometimes (such as nero) can be removed from the startup list without any harm (meaning they won't start every time you turn on your PC). Some programs can turn them off for you but Google is your friend for these kinds of tutorials if you don't know how to do it yourself.
I really hope this helps! I might have missed out some details but it's meant to be a really basic guide anyway. Feel free to tell me what I should add in the comments.