Samuel Adler's 'The Study of Orchestration' is a classic. It comes with a cd and a workbook. In addition, listen to a lot of music while following it in the score. On imslp.net you'll find tons of free scores (bascially everything where the composer's death has been more than 70 years ago, meaning it is no longer under copyright). For example, download the scores for all the Beethoven symphonies. Then listen to them following the scores. For the parts you like the best, go back to the score and read very carefully how they have been orchestrated. It's fun and quality time spending half an hour listening to a symphony like this.
Then, go to orchestral concerts regularly. If you can, try to attend rehearsals of a local orchestra. Maybe there is a student orchestra in your region, and you know somebody who plays there? Ask if you can attend a few of their rehearsals and listen quietly. Maybe you can make yourself useful and help preparing the seats and music stands before the rehearsal in return. Or maybe there is a good youth or amateur orchestra you could ask. Attending rehearsals will be enlightening, as only there you'll learn how orchestras work behind the scenes. This will be priceless knowledge for any composer.
One final idea, search youtube for videos of famous conductors rehearsing with orchestras. There are a few with Nikolaus Harnoncourt and Bernhard Haitink. I'm sure there are lots more. Again, follow these with a score/scores of the piece(s) they are rehearsing.
That's half of what you can do to learn how to orchestrate well. The other half is just doing it. Orchestrate, orchestrate, orchestrate. Write music, orchestrate it, and have somebody correct it (if you don't know anybody you can post it on this forum, for example). Noteperformer is a valuable tool that will give you aural feedback of what you wrote. While it doesn't sound like a real orchestra or even a good mockup, it is very good at making mistakes in orchestration clearly audible.
Another trick: go to imslp, choose an orchestral piece by Mozart, and download the piano reduction only. From the piano reduction, orchestrate 8 or 16 bars (without having looked at the full score!). Then download the full score and compare how Mozart orchestrated the same 8 or 16 bars. I promise that you will learn a lot from this exercise. Repeat as often as manage, and progressively choose later composers music as well.
While orchestrating I found this reference chart to be useful for quick checks of ranges and instruments capabilities:
https://www.dropbox.com/s/wqepbz9jx8555ap/InstrumentReferenceChartv6.1.zip?dl=0 (chart not made by me, I'm just linking to it)
If you need more indepth information, you can always read up in Adler about a particular instrument or technique.