The most common software used in creating conference posters is Microsoft PowerPoint. The conference guidelines should state what the dimensions of the poster should be, the PowerPoint slide can then be set to this size.
It is also possible to create posters in other software including Microsoft Publisher, Adobe InDesign and Canva.
When designing your poster you will need to read your chosen conference's guidelines carefully. Ensure that you have used the conferences suggested orientation (landscape or portrait), and follow any other guidance such size, word count or referencing style.
Before designing your poster on a computer sketch it out on paper. There should be a clear flow to the poster and it shouldn't be too full of content. When designing ensure that there is enough empty space.