Uneven (or tilted) ground inside the shed
 

Hello,
I am here looking for advise (as I don't seem to be able to find what I want from my DIY book). Hope someone can help me. Thanks!

As a first time home-buyer, I do have tons of questions. But, I'll start my first few questions about my garden shed.

The ground of the shed seems like made of concrete, but the surface is very rough, uneven, and maybe tilted from one end of the wall to the other end.
I wonder how i can make the ground even and hopefully smoother before i even try to do the flooring. Like, should I lay another layer of concrete or anything? If yes, what and how?

Another question:
The 4 walls of the shed are not completely sealed (or attached) to the ground. At some places, there can be 1-2 inches gaps. Does it affect how i lay an even ground?
(Note 1: I guess the walls have insulation (with sth like yellow sponge).
Note 2: In rainy days, there are a few spots where water can come in and stay along the wall until it dries.)

Thank you in advance!