Can this picture be considered as decent? - 1
I went to a football training last night and I tried my best to shoot the best I can using my Nikon D5100 and kit lens (18-55mm) combo.
The colour of the picture was edited. Also, the original picture was much bigger and you can't barely see the face of the players. This picture in the link was cropped so it was kind of like zoomed in and the players appear much bigger in size.
Added (1). Also, do you know any way (converter, software or something) that allows me to zoom in the picture I took without reducing the size of the original picture (like I won't have to crop the picture)?
If you read a good photography guide… You won't need to ask 'basic' questions like this, i'll give you some credit because you have actually tried using your camera… An 80-200mm zoom lens or something similar is what you are looking for… It's difficult to get good shots in sport and takes a lot of practice… So if you do spend your money… Put the effort in to get the good shots… Patience and effort reap benefits in the long run… Stick with it and you will get there.
A converter to double the reach of the lens would lose you 2 f-stops, so your 55mm lens at f5.6 would become a 110mm f11 - no good for action photos. Bite the bullet and get a tele zoom, maybe 55 to 200mm or similar.
Sorry, I couldn't open the link, so can't comment on the photo. And, as others say, you really need a bit of general knowledge about the technicalities of photography.
As a photo to show the atmosphere in the stadium it's OK. Just consider cropping 1" from the Right and 1/2" from the Left so as to keep the eye from wandering out at the edges.
Teleconverters are really only OK when added to fast f2.8 expensive lenses because they will steal at least 2 stops of light and make you increase the ISO (resulting in digital noise) in order to keep a fast enough shutter speed to trap any action.
Consider getting this Tamron lens, the SP 70-300mm F4-5.6. (see link)
Be careful when getting older 3rd-party manufacturers lenses for Nikon as they may not function fully.
The picture is descent
You can't zoom in without cropping. As others have said, a converter will reduce light and make the lens too slow. A proper tele-zoom lens is what you need. The Nikon 70-300mm VR ED lens would be great if you can afford it. That is a superb, sharp zoom and it is fast to autofocus. Anything with a much wider aperture will be very expensive.