I just started Unity and want to create a 2D platformer, but I need a bit of help, such as:
How to change sprites and textures?
How to program “spikes”?
How to program levels and a level select screen?
How to create moving objects?
And some other questions.
Anybody able to help me on this?
I have three very general suggestions.
One, be sure to check out the Unity Learning section on this website (as noted just above). They have scripts and videos about the many features of the Unity engine. Also, since you will be writing code, check out the online manual.
Two, maybe dissect a free game (or two, or three) that you can find in the Unity store. Make small changes here and there to the assets and to the programs and see how it works out. I’m not sure what exactly you would be interested in, but maybe this one would be close?
Three, if you’re looking for an idea on how to handle something in script, or an explanation you’re not finding on this official website, there are a lot of Unity resources out there on the wild web (including C# scripts, shaders, tutorials, and so on), and Google can help you find them. I bet there are probably a few tutorials on YouTube about making platform games with Unity.
(BTW, you should automatically be checking out things like this and this, as they were made for this purpose.)
Good luck! 
Thank you so much!