Do you roll for who goes first and then decide bf?

No, that is incorrect. According to the official rules, you must determine the battlefields before deciding who goes first.

Here is the correct Start of Game sequence:

1. Reveal Legends and choose Champions.
2. Determine and place Battlefields: Battlefields are placed face-up in the Battlefield Zone. They are public information as soon as they are placed.
3. Determine turn order: Players use a fair random method (such as a dice roll) to designate a player.
4. Choose play order: The designated player chooses whether they wish to go first or last.
5. Draw cards and resolve mulligans.

As noted in the RiftJudge FAQs, while some players mistakenly hide battlefields until after determining turn order, battlefields are public information as soon as they are placed and must be revealed before anyone makes the decision to play first or last.