The Trial Key allows you to play the game for 7 days. To continue playing after that, you will need one of the Starter Packs.
Trial Keys can be purchased in the game, so the only way to receive one is to get it from another player.
First download Albion Online, install it, create an account and log into the game. Then click on the 'Redeem Code' button and enter your Trial Key code.
You are unable to interact with the gold:silver player exchange market, you cannot buy premium and you cannot progress beyond Tier 4. But once you upgrade your account, all these features will be unlocked.
Once in the game, go to Settings > Invite Friends