how to select my speakers

Are the computer speakers active or passive (do they have an amplifier=active?) If they're active, I think it's better to connect them to the line outs of Focusrite.
If you want somewhat decent sound quality you need to spend around 200 dollars at least, IMO, for beginner level active monitor speakers.
The cheapest way to nice sound quality could be buying a 15 years old hi-fi set (amplifier and speakers).
 
Good headphones are a better alternative than small PC-speakers. You just need A LOT of trial and error and use of reference tracks and an analyzer to learn to record/mix/master so that the result sounds decent on any music system. That trial and error you'll need with the speaker as well, though.

Here's a different approach.  Set your existing system up the same as though you are going to do a mix.  Playback an audio file and set the loudest part of the audio file to a comfortable listening level, not too loud or quiet.  Whatever level you choose, you need to start thinking of it as the level you will be mixing at for the majority of the time you are editing and mixing.
 
Now that you have determined your mixing level, head over to YouTube and search for "hearing test".  There are several videos that do constant level frequency sweeps from 20 to 20,000 Hertz while displaying the frequency so choose one.  Run the video (you may want to run it several times) and listen through your system with your system set up to do a mix.  You will discover three important pieces of information:  the lowest frequency you can hear, the highest frequency you can hear and several frequencies where the apparent volume level will change.  Now you can adjust your mix incorporating this new information.