Agreed.
Start with "The Study of Orchestration" by Samuel Adler. If you can't afford it or steal it, check the numerous FREE nooks and crannies on the internet dealing with melody, harmony, rhythm, and, oh yes, orchestration. Write lots of pieces with a solo instrument. Then with two. Then three, etc. Work your way up to the orchestra.
If you don't do it this way, by all means write for the orchestra and experiment. See what works and what doesn't work to your ears. Once you create some glorious monumental Mahlerian work, feel free to post it here. Members here can tell you all the things that aren't working, which no offense, will be most of it. You'll get the same advice of my comment and most everyone else's above mine, and you'll have wasted time.
Just sayin'.
I feel like a topic like this crops up every 2 weeks or so. :/