Tawalla means to love and follow the teachings of the Prophet (pbuh) and his Ahlulbayt. However, it is not enough to express the love only through words but the words must be coupled with actions, i.e. following the laws and guidelines laid out by them and being available whenever needed.
Tabarra means to keep away from those who are the enemies of Allah, His Prophet and the Ahlulbayt. It also means keeping away from those who do evil.