i am creating a 3d environment and i know that highpoly models have to be optimized but i dont know how much ?
This is not a big deal and you shouldn’t really worry about it, as long as poly counts are not ridiculous. The ShooterGame player model has 16k tris and you can easily get over 100 FPS in that example, but i’ve seen character with 50k tris work fine. My VR project has 5-10k polys for every gun in it, and several guns can be seen at once in the level.
Long story short, just keep tris reasonable for what you’re building. Characters and weapons will be seen close, so keep polycount high for these sorts of items, and keep polys lower for props and things that won’t be seen up close, or in detail.
Hope this helps 
It all depends. At first, it’s best not to worry about it too much, but at some point you may find yourself needing to do a pass and optimizing things to get more performance. It’s a balance, because as a single dev, your time is one of your most valuable assets, and optimizing more than you have to can be a real hassle. Having said that, it IS a good idea to follow good topology practices with respect to edge loops, flow on your models so that you CAN easily come back and create a lower poly version easily.