I'm developing an application and my friend wanted to join the development. So I created a git repository on assembla.com , performed a push and invited him to work with me.
My question is about the working proccess. We both use git bash on msysgit. I don't have problems with commit and push, the problem is with getting my friend changes. Why a simple pull does not work?
Short answer (judging from the comments):
git pull origin master
If that works as you're hoping, make it the default by adding this to your .git/config file:
[branch "master"]
remote = origin
merge = refs/heads/master
With that setup, you can then call:
git pull
...in future.
Long answer: It sounds like you haven't set up default merge targets.
You want a configuration setting that says, "Whenever I pull from remote repository X, automatically merge the changes with branch Y." (Typically, "When I pull from 'origin', merge with 'master'.") That's automatically set up when you do a 'git clone', but your working copy was created from scratch (not cloned) so you have to do it yourself.
You might find it interesting to compare your .git/config file with your friend's.