I want to change permissions for some folders, I know chmod is used for that, but I don't know how is it used, man chmod doesn't do any good to me, I don't understand a thing it says... What do those numbers, for example 755 and so on, mean?

Always try a search engine first. google.com and ask.com seem to be the best. here are the results i got for searching "What do the numbers in the chmod command mean?" on ask.com; might be worth a look.http://web.ask.com/web?q=What+do+the...%3F&qsrc=0&o=0

the 3 numbers represent ,from left to right, permissions for the owner, group, others respectively. A 1 represents that the file is executable and for directories it means you can access it. A 2 means you can write to the file or directory. A 4 means you have read access and for directories you can display (ls) it's contents. A 5 is 4 + 1, a 6 is 4 + 2, 7 is 4 + 2 + 1. Each means the combination of what make them up. Eg. 6 means read and write.

actually if you are really into it, these are 3 octal digits (base 8).
In binary
1 is 001
2 is 010
4 is 100
5 is 101
6 is 110
7 is 111
..... rwx

Ah the numbers

The mysteries of Chmod explained:
basic syntax:
chmod 777 filename
what it means:
first number = User (what permissions do you want the user to have)
Second Number = Group
third number = Other (everyone else)

Now what the numbers mean:
0=No permissions
1=x (execute)
2=w (write)
4=r (read)
Then you can add the numbers so if you wanted to do more than one like:
full permission
rwx=4(r)+2(w)+1(x)=7
Read/Write
rw-=4(r)+2(w)=6
Read/Execute
r-x= 4(r)+1(x) =5

So chmod 765 junk would do the following:

blink@sanity ~ --> chmod 765 junk
blink@sanity ~ --> ls -l junk
-rwxrw-r-x 1 blink blink 3 Sep 23 12:15 junk
blink@sanity ~ -->

Of course I always use the letters instead; works just as well and easier to use. eg
chmod u=rw file
sets user perms to rw (read/write)