Your message can be the same one as you wrote above or :
"Hi Teacher, I am new here. My English is poor (A1, for instance). How can you help me improve it to A2? or tell them your goal". See, Oliver, you want to talk to teachers first and not just choose it randomly because you'd want to invest your time and money wisely being with those who have your interest at their heart!
I hope it helps. Italki is an amazing and friendly community and you'll enjoy the process of improving yourself. Finally, if you'll love it as I do - share it with friends as my existing students do because of the value they receive by studying here.
Be well and have fun in the process!!!