Can I make it sound realistic?

Making a synthesized instrument sound realistic is an *incredibly* difficult problem. There just so much more to it than just having a good soundfont. A violin is capable of sounding very different depending on how you attack the strings with the bow - bow speed versus pressure, when you change bow direction, where on the string you bow, how much vibrato you impart with your finger, etc. Real music happens when a human musician controls all of these variables for each and every note. And of course, different violins sound different, and different bows do too. Similar issues for most other instruments.

So expecting miracles is just not realistic. There is software out there - digital audio workstations, they are called - that can give you this sort of control over the playback, but you have to work just as hard to control these variables to really get the most out of it.

That said, Finale is capable of sounding very very good, but not with the default sounds it uses - you have to install and set up the "Garritan" sounds, which is a lot of work and takes a lot of computer resource.

And while GeneralUser has some good sounds, it definitely shows the limitations that are inherently in any soundfont that makes compromises to get as small as it is. There just are not enough samples, nor are they long enough, to get results that sound as good as larger soundfonts such as the Garritan library (which is not available in the format MuseScore uses), or even FluidR3 (which is available, and free). There might be individual things in certain contexts that can sound as good in a small soundfont like GeneralUser as what you get in a larger soundfont, but overall, bigger really is better, and that's why FluidR3 really does work better overall even if you a brief trial might make you think GeneralUser is as good. But still, even FluidR3 is nothing compared to Garritan.