Being somewhat new, I want to use this lense for landscape/wildlife and nightscape/stars. i was told to also look at the Sigma 18-35mm f1.8 dc hsm lense. what one would be best for what i want to do?
You've got 3 very separate requirements here: Landscape, Wildlife, Nightscape/Stars. Landscape and Wildlife are only similar in that they tend to reside in the same places, but the right lens (and sometimes the right camera) differ greatly. Lets lay out what your typical lens might be for each, remembering that when shooting with the crop sensor on the D3400 the lens' focal length is effectively 50% greater (your 70-200mm looks like a 105-300mm on a 35mm or FX body). When I mention focal lengths below I will be speaking in the numbers printed on the lenses as if you're shooting in full frame but will make comments as needed.
Landscape & Nightscape/Stars: In general you want a good zoom that will cover everything from ultra-wide to normal vision range, from 14mm or 16mm to 50mm. The 18-35mm Sigma is an effective 27-52mm zoom on your camera and in general will work really well for landscapes. Nightscapes tend to be shot rather wide and you want something that will let in a lot of light, so the 18-35mm with its f1.8 fixed aperture is actually a great lens for that type of work. So with that lens you've got 2 of 3 areas covered. As you shoot you'll learn whether or not you wand to go wider than the effective 27mm that the 18-35m will give you, so you might start looking at some of the ultrawide zooms out there. I have long used the 8-16mm Sigma for DX cameras, but I know others use the 10-20mm from Sigma as well.
Wildlife: This is where reach/length is key. A 70-200mm is a great basic wildlife lens that work extremely well when your wildlife isn't bird sized. If you're looking to shoot deer-sized critters and larger this will work well. If this is your passion, as it is with me, you'll likely soon find yourself cropping in more than you'd like and winding up with really small bits of your subject, so eventually you'll be looking at something longer (100-400mm, 200-500mm, 60/150-600mm) but you'll surely know it when you get there.
So, with all that said, those two zooms will hook you up for those styles as you start your journey. That said, if you don't think you'll be doing a lot of night photography then you might want to consider a zoom that covers all the space up to the 70mm on the low end of the telephoto. I have an 18-105mm Nikon kit lens that I use all the time on my D7000 (converted to IR) and it's the perfect walkaround lens and you can find them used relatively cheap. Sigma makes a 17-50mm f2.8 and a 17-70mm f2.8-4 Macro lens (both are DX specific) that might serve as a better alternative if you're only getting 2. If astrophotography is something you really want to explore then I suggest sticking with the 18-35mm and getting a 50mm prime as it both fills in the gap and will give you an opportunity to explore shooting with a single focal length, something I highly recommend doing regularly.