You speak for a state, not for yourself. Every speech, every clause and every vote is your country's, and it has to be consistent from the first day to the last.
Look for what the country has actually said and done, in this order.
Statements at the United Nations. Speeches in the General Assembly and in the Security Council on your topic.
Voting record. How the country voted on past resolutions about the same issue.
Treaties. What it has signed and ratified, and what it has refused to sign.
Government sources. The foreign ministry, the head of state, official press releases.
Alliances and blocs. Who it usually votes with. Regional groups and economic partners.
The situation at home. Its economy, its geography, its own experience of the issue.
Where these disagree, prefer the most recent official statement, and be ready to explain the change.
If you are unsure of the position on a narrow detail, work from what the country has voted for and what it has signed. If there is still nothing, take the position closest to its stated interests, and be able to say why. A defensible answer built from the record is better than silence.
Your position paper, your speeches, the clauses you write and your votes all have to point the same way. Delegates and chairs will notice when they do not.
If your country's position is unpopular in the room, you still hold it. Arguing your country's case well is the point of the exercise.
Representing a position is not endorsing it. You are not being asked what you think. You are being asked what your country thinks.
Find the delegations whose position is close to yours before you start writing. That is what lobbying is for. See The lobbying process.
Real alliances matter. A bloc that would never exist in the real world produces a resolution that would never pass in the real world.
Say "Less Economically Developed Countries". Do not say "Third World Countries".
Speak of other delegations in the third person. "The honourable delegate" or "fellow delegates". See Parliamentary language and decorum.
Not every delegation is a member state.
Non-member delegations, such as non-governmental organisations and United Nations organs, have full voting rights. They may vote on amendments and on resolutions like anyone else. See Who may vote.
In MEDIMUN, delegates in a Special Committee usually represent an entity rather than a country. Research it the same way. What it is for, what it has publicly said, what it has the authority to do, and who it answers to. See Special committee delegate guide.
If you represent a permanent member. The five permanent members are China, France, the Russian Federation, the United Kingdom and the United States. Each has a veto. The veto is almost never used in reality. In 2013 the Security Council passed 47 resolutions, 43 of them unanimously, with no vetoes at all. A delegate representing a permanent member has a responsibility to reflect that. See The veto.
Non-permanent members are expected to reflect real alliances and real economic ties. Blocs organised against the permanent five are discouraged and should draw a warning from the chair. See P5 and non-permanent members.
In MEDIMUN, one delegate represents one country in a committee. THIMUN seats two delegates on one country in some of its forums. MEDIMUN does not do this.
In the plenary session. Every delegate representing the same country sits together and speaks as one delegate. The country has one vote, whoever casts it.
Write the position down in one page. The topic, what your country wants, what it will not accept, who supports it, and the two or three facts you will use most. Bring it with you. When you are put on the spot in committee, that page is what you will reach for.