This is my first attempt to create a tutorial here. The tutorial is targeted at those with little or no experience with pixel art. I will also try to share some things that worked for me along those years. Those are my personal views and if you want to take it more serious, there are several very good and complete other tutorials out there. So let us gets started.
If you are giving the first steps I would say the tool is not very important (if your wish to make something is). I started with MS Paint and sometimes I still use it if needed. In any case, the tools I consider useful for me at the moment are Aseprite and Pixaki (for iPad).
Start with basic shapes and try to get the overall picture rather than details. It does not matter if its perfect as it can always be refined later. In the case of football, let us make a basic standing player using Puyol as a model.
As you can see, I used basic squares to break down Puyol's body part and then I drew those individually (you do not need to draw them individually, but this should give an overall idea of the process). Next, I just adjusted some of the borders to give a better appearance to neck, hand, etc. This give a silhouette and we can now go to colouring.
Choosing the colours is one of the most difficult steps in my opinion. I strongly advise you to start with a fixed colour palette (if you want to get nice results faster). In the website Lospec you can find many different palettes. My favourite is Endesga 64, by @endesga. If you wish to pick own colours, my tip is: forget RGB and use HSV. It might seem strange if you see it for the first time but it is more intuitive since the H (hue) part is the actual colour you might be looking for. Back to Puyol, you can see that I had applied some basic colours to skin, hair, shirt, short, socks and boots. In doing this, use as many references as possible.
This is the big step in the process: to polish the image to get to the final results. There are not many rules, here but I will try to outline the parts I think are more important. The key to this step is the observation (which is also a skill by itself).
Player specifics: If you look at Puyol, what stands out? Of course, it is his long curly hair. The trick was then to alternate the darker and lighter pixels to make it look like curly. Jersey details: Do not worry about each detail of the jersey. You will never be able to replicate it exactly (in such a low resolution). Just try to make something that will resemble the original 'in average'. Shading: You can some darker pixels between his legs and under his arms. This is to add a bit of 'realism' as those areas are likely to get less light (and therefore are darker).
I know we all want to make very beautiful pieces in our first attempt, that that is not likely to happen. So, if you really want to improve, you just need to do it again and again and yet again. If you see the first drawing I made, you will think it was made by a 5 year old child (I could not make such a thing when I was 5). The point is that you will get better as you do it repeatedly and you will not even notice. I leave other examples here hoping to inspire you: