You'll have to purchase italki Credits (ITC), which are virtual currency, from italki.com. italki Credits are used to pay for foreign language tutoring services rendered on italki.com. Then when you have completed a lesson with a Teacher, italki.com will then transfer the ITC to the Teacher's account. (Refer to Student Guide)
If you trust italki.com then there is no problem as all payments are through them. In fact, many members here have already benefited from the tutoring services. You may ask any members if they have been cheated, just to be sure.