Hi Cynthia, welcome to Tycoon Talk!
1. How do I explain to developers what I need? do I need to create mockups? how detailed it should be?
Developers are coding guys, not designers. A mockup of what they need to do would be helpful, but more in a form of a flowchart. Tell the developers what will happen if the user does
something.
2. Do they also take care of the design? do I need to provide them with pictures and logo?
Again, devs aren't designers. Comparing PHP/MySQL (standard development stuff) to Graphics & CSS is like comparing apples and oranges. Most developers can't design to save their life, just take a look at some Google apps. Most are the same layout, same colours, and overall just boring. However if you look at a professional designer's portfolio, it's usually amazing.
3. Since I am a developer myself, will I be able to see the code (partly ready) during the development and provide my comments. Maybe when each milestone is over?
If you tell the developer to keep SVN repos or GIT-things, then probably. Also, if you develop on a cloud, it'll make keeping track of the current code trivial. You could also just tell the devs to send you periodic updates on the current code.
4. just wanted to double check that I will I have the full rights for the code?
That depends on whether or not the developers will just *give* it to you. If you pay, you still might have licencing to the original creators. For example, if you buy a movie at Wal-Mart, the movie itself still belongs to Dreamworks or Disney. However, I'm not 101% sure about the freelancer.com licence policy, I hang out on the forums
5. they will hand me over the code (not fully completed) after completion of each milestone, right?
I suppose, if you tell them to. Developers are humanoid creatures; you can speak to them and they can speak back

. If you ask for a milestone, then they might. However, some developers might be hesitant because you could just run off with their code without paying an up-front milestone fee.
6. are there any tutorials or tips that I can read?
Tutorials for freelancer.com? You could read
this,
this,
this,
this, or
this (just going through the 'help' dropdown).
As a side note, why are there so many people wanting Groupon clones? It's crazy, scrolling down the list of jobs to do, seeing at least one Groupon-clone-type-thing per page.