I'm trying to make midi tracks, with the on screen keyboard.

All the sounds of strings are terrible. Is there anything to add to the program to have better midi sounds?
Which versin of ACID Pro do you have? ACID Pro 7 is the current version, and it comes with Garritan Aria for ACID Pro Player, which contains some of the sound from Garritan Personal Orchestra. It's not the best orchestral sample set on the market, but it's not bad, especially in skilled hands.

If you have ACID Pro 7, then you have the Garritan Aria Player. If so, I'll help you figure out how to open a MIDI track, open Garritan as a virtual instrument, and assign the MIDI track to Garritan.

Also, there are a few free Soundfont players out there that you can open up in ACID the same way, and there are ton of free soundfonts to be found that range from terrible all the way to pretty good.

Here's one of the soundfont players called SFZ. Just install it, and ACID Pro should find it.

SFZ Soundfont Player
Yeah I have Pro 7, Sorry I forgot to put that.
Some help would be great
Click "Insert Soft Synth," it should be somewhere near the bottom right. Select Garritan Aria. In Garritan Aria, select the sounds you want, like violins, cellos, etc...

Insert a MIDI track, then assign it to Garritan Aria, and create a melody line on the MIDI track and it should play through Garritan.

Then you can export that as a wav or do whatever you want.