For undergraduate research conferences business attire (suits, ties, dresses, dress pants, collared shirts, and similar clothing in good condition without tears or stains) are a typical expectation. As you get more established in your research discipline you may find other attire is common, but a good general rule is to dress more formally (to be "over-dressed") than not dressed formally enough. Jeans are considered too casual for this setting.
You will likely repeat the same 2 to 3 minute speech multiple times during the period that posters are being presented. Determine which brief highlights of information explaining your research you wish every audience member to know. A great way to prepare for this is to consider how you would briefly explain your entire research project to a friend without losing their interest. These short speeches are sometimes referred to as "elevator speeches," because the expectation is for the presenter to provide complete overview information within the short amount of time it would take to ride an elevator from one floor of a building to another. These speeches act as an introduction to or summary of your research, the poster text will provide the finer details, and you as presenter are there in person to supply further details and answer questions as needed.
Some audience members may want to only read posters and not interact with presenters. This is ok! Smile, be present and attentive, and indicate you would be happy to answer any questions they may have.
Depending on your research project, you may wish to have a copy of the following with you to share during your poster presentation:
The night before presenting a poster, review your research outline/paper and perhaps the most important reference you cited so that the information is fresh in your mind. Being able to quickly recall most details of your results or findings when asked questions by the audience increases your perceived credibility. If you don't know the answer to a question or hadn't considered something introduced by an audience member, that's ok! Be sure to respond kindly and honestly. This can signal a moment of exchange where the audience member may wish to share additional information or ideas with you, and where the real conversation about research begins.