Bináris,
to claim or assign a task, you have to go to a particular task, then at the end click on the "Add action..." list, then "Assign / Claim" and write your account name or someone else's (it will autocomplete). Please note that assigning a task to someone without any previous interaction or context is generally considered "bad etiquette" by most people or teams, unless so documented.
A good place to search for Phabricator help is[0] and to ask questions, [1] (there are versions in other languages); I encourage you to ask there further doubts. :-)
Regards,