After you have created an AnkiWeb account, you’ll want to then “sync” your computer’s Anki flashcards with AnkiWeb.
Remember that when you "sync", it means that all your files are copied via the internet from your computer to your online AnkiWeb account. A “sync” can also mean that your flashcards that were saved on AnkiWeb can be copied from there to your computer Anki or Anki on another device.
Syncing your computer Anki’s flashcards with AnkiWeb for the first time
Click on the sync button in Anki. Make sure you are connected to the internet whenever you click the sync button.
After you click the sync button, an “Account Required” popup will show up. Since you already have an AnkiWeb account, type your AnkiWeb ID and password, which you created in the signup process. Then click “OK”.
The merging conflict error popup during the first sync
When you synchronize your Anki with AnkiWeb for the first time, a popup will display and Anki will ask you if you want to upload to AnkiWeb or download from AnkiWeb. The popup will look like the one below.
The reason for this merging conflict error during the first sync
The reason you see this error popup is because you already have flashcards on the Anki on your computer. However, you created a new AnkiWeb account but there are no flashcards yet on that account on the internet.
Since your computer Anki and your AnkiWeb differ so much, it is difficult to sync the account at the moment, so this popup comes up to ask you which one is the correct version of your flashcards. This conflict problem is essentially the problem that this popup is describing, and this is normal after creating a new AnkiWeb account.
Solving this merging conflict error during the first sync
If you just created just AnkiWeb account, then there are no cards at all in that AnkiWeb account. Therefore, the correct versions of your flashcards are the ones that are on your computer Anki.
As such, you should click the "Upload to Anki" option. The reason you'll click this is that then all your flashcards from your computer Anki will be copied and backed up onto AnkiWeb.
Warning: Do not click "Download from AnkiWeb" during the first sync, otherwise your computer Anki will get all the flashcards from your AnkiWeb account. Since your AnkiWeb account is new, then there are no flashcards in that account, and so downloading from AnkiWeb will replace and wipe out all of the flashcards on your computer Anki.