You can only remove a condition if you have a spell or ability that lets you do that. For example, the Purify spell lets you remove poison conditions like Rot, Weak, and Cripple. The Priestess can remove any condition. In these cases you must pay the removal cost of the conditions you choose to remove.
Future spells will enable you to remove other conditions. For example, Clear Mind will remove all Psychic spells and enchantments, such as Sleep, Confuse, and Mind Control (the latter 2 are new stuff!).