Sound Transit tells us that there will be lines, and you should be prepared to wait. They expect from 50,000 to 100,000 riders to ride Link that weekend. (To give you an idea of how big that number is — the population of Seattle is 602,000. So potentially one in six Seattleites will ride Link on opening weekend.) Because of the large crowds, South Lander Street will be closed to vehicle traffic between Beacon and 17th; it’s been closed for several years already for construction, so we should be used to it.
If you need to ride the bus somewhere during that weekend, be aware that buses will not be using the tunnel, and tunnel buses will be on their surface routes instead.
Though rides opening weekend are free, after that you will need to pay. Consider getting an ORCA Card soon. You can use the ORCA on the bus, train, and ferry. (You can still pay with cash, but you’ll have to buy tickets in the train station. There won’t be fareboxes on the trains.)
A bit of station trivia: Each station has an icon, or “pictogram” that represents the station and its area, visually. Beacon Hill Station’s pictograph is a kite, representing “a sense of light and air, as well as community spaces and summer picnics, items that play prominent roles in Beacon Hill’s neighborhood plan and history.” See more about the pictograms and their meanings in this PDF from Sound Transit.