Please make sure you have allocated 3GB-4GB to the launcher, and are running Java 8! If that does not resolve your issue, visit our Discord and we will try to help you.
Type in the modpack name (Galaxy-Craft Lite) or paste the following url into the search box.
Finally, click Install at the bottom right of the launcher after you select Galaxy-Craft Lite from the list on the left. The launcher will handle everything else!