Select the Friends tab and choose Join Realm. If you are playing on console, enter the 6-digit invite code. If you have received a Share Link invitation, the invite code will be the last six digits of the URL.

Where do I find my realm invite code?

Go to settings/members and in the top right you should see the "share link" button. Copy the code and there you go. You can also invite friends from the menu.

How do I find my Minecraft realm invites?

Launch the game. On the top right of your window, you'll see an envelope button. There you can find all your invites to different Realms and accept the ones you want. Once you've accepted an invitation to a Realm, click Play and the Realm will appear in your Friends tab under the Joinable Realms list.