First, attend social situations that don't require much effort from you. Events such as a comedy show, book reading, or concert are ideal. Once you're comfortable being in a crowded environment, engage in more intimate occasions that will require your direct input.
Focus on baby steps as you learn to feel comfortable in social situations. If it's been a while since you've interacted with more than two people at once, it may feel overwhelming to go to a party where there will be many guests.