Alright somthing i know a little about.
First as everyone has said all you computer has is a signal output(even with a good sound card no amp) i think you got that.
So you are trying to build a set of self contained speakers and will take the output from your coumputer. So what size. I would recomend some 6inch mid bass speakers with some good tweeters. 2 way design
So for the speaker project
1. decide what speakers you wont to use.( if there going to be near your coumputer use sheilded speakers or you can mess some stuff with the magnets).
2, Build or buy some pre made enclosers for the speakers.
3. design and build your crossovers, or buy them.(this is a electrons forum build them) do a google search on how to design and build crossovers and go from there. plenty of info.
4. buy or build an amp. or use an old reciver,stereo ect.
Here what i would recomend.
Good dayton sheilded speakers $22 each
https://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=295-306
Tweeters good tweeters $6 each
**broken link removed**
corossvers $11
**broken link removed**
Then amp 40 watt stereo amp kit. 10Rms per chanel.
https://images.google.com/imgres?im.../images?q=home+stereo+amp+kit&um=1&hl=en&sa=N
All you need to do is build you some cabnets for the speakers and mount the amp in one of the and plug it into your counputer.