目的 and 目標: purpose and target
Distinguish why an action is undertaken from the target it aims to reach.
What the action is for
目的 is read もくてき and refers to the purpose of an action. 「旅行の目的は友達に会うことです」 says that the purpose of the trip is to meet a friend.
「目的をはっきりさせましょう」 proposes clarifying what the activity is for. It can be used to establish its direction before deciding on a frequency or score.
What point to aim for
目標 is read もくひょう and refers to a state or standard you aim to reach. 「今月の目標は本を二冊読むことです」 says that the goal this month is to read two books.
「目標を達成しました」 states that the goal has been achieved. A 目標 does not have to be numerical; a desired way of being can also be a goal.
Connecting the two in a plan
「会話力を伸ばすことが目的で、毎日十分話すことが目標です」 distinguishes improving conversation skills as the purpose from speaking for ten minutes daily as the target. The target is chosen to serve the purpose.
In context, 目的 and 目標 may refer to similar desired achievements. Rather than enforcing an abstract-versus-concrete rule, consider whether the sentence emphasizes why or what point to reach.
Does 「目標を立てました」 mean that the goal has already been achieved?
Show the answer and why
No. It means that a goal has been set. Achievement is 「目標を達成しました」, so distinguish setting the plan from completing it.