For someone who says they're a beginner, this is a cracking shot!
You've used a fairly wide aperture (f5.6) which has given you a limited depth of field (the bit in front of and behind the main thing you focused on). This has the effect of directing the viewer's attention firmly towards the main subject, rather than wandering endlessly around an image that's sharp throughout. In your image above, the sharpest point of focus seems to be on the little ferny fronds in front of the mushrooms (or toadstools?) ... and while the fronds are beautifully sharp, I think the mushrooms (as the main subject) should be the sharpest point. (Mind you, it is only fractionally out, so it's not too bad
You've chosen to use a 'Daylight' white balance, rather than leave it to the camera to decide - and while this has given the image a slight warm cast, I think it suits the subject well.
Your ISO of 1400 has given the image quite a grainy look (although not as much as I would have imagined ... Nikon must be good!) - the higher your ISO, the more 'graininess' you'll see in an image. If you want clarity and quality, you'd be better off using an ISO of 100 or 200 ... this may mean, though, that your shutter speeds get a bit longer as a result, so using a tripod is definitely the way to go for this kind of image. Okay, you may have already done that, but given the shutter speed of 1/15th second, if you didn't use a tripod, then you've got very steady hands!
You've shot in JPEG mode ... and I would also hazard a guess, from the very saturated colours in your image, that you've got some sort of image enhancing mode turned on in the camera settings themselves? Something like vivid colour mode? I know you said in your earlier forum post that you're looking at post processing and editing software, so I don't know whether you've already got something that you're using, or whether you're just relying on the camera's picture settings at the moment ... once you get into the editing side of things, if you shoot in RAW format (the one in the camera; not where you have to take your clothes off
) and then edit the RAW file in an image editing software, you'll have much more control over how you want the colours to appear in the end result. JPEG is great, but it also throws away a lot of information once it's taken the image ... you don't necessarily get the choice afterwards to make the editing decisions you'd like to.
The other thing I'd say is that you've placed the main subject (the shrooms) dead centre in the image ... while this is an easy thing to do, and seems to be the best position, it's often the most boring. Try composing with your main subject off to one side a bit - it'll make the end result far more dynamic and appealing to the viewer. It's not always possible, depending on what's to either side of your main subject, but if you bear that in mind when you're composing your images, then you'll do yourself a favour. (Having said that, sometimes something will look far better placed dead centre ... but also this often looks better when in a square framed image, rather than the landscape format you've got here (i.e. longer than it is high).
Okay, if you're still awake at this point ... I really like your image. The colours zing, the tiny ferny fronds are amazingly beautiful and to be honest it's a damned good start to your photography journey!
Keep it up