So you managed to bypass the maintenance screen of SINoALICE, and with the first thing made available to you being the free 10 roll screen, here is a quick guide on how to go about rerolling.

What To Aim For

Data is still being gathered at this point, but the best thing to roll for now are weapons that also unlock jobs for your characters. The following is what is available for the time being:

Staff of Restriction (unlocks Alice/Cleric)
Blade of Depravity (unlocks Cinderella/Breaker)
Violin of Delusion (unlocks Hansel Gretel/Minstrel)
Grimoire of Dependency (unlocks Pinocchio/Sorcerer)
Staff of Suffering (unlocks Kaguya/Cleric)

If none of the above appears in your roll, the next step is to clear your data in order to initiate the process of rerolling.

Android

Unrooted

Clear app data (Settings>App>SINoALICE>Storage>Clear data)

Rooted

Delete the Shared_Pref folder inside of data/data/jp.co.pokelabo.sinoalice

iOS

Simply delete the application and download it off the App Store again.

Post-Reroll

Once you are happy with your roll, simply proceed with the game where you are prompted to download additional data. Eventually you will reach the main menu where you will probably want to bind the account to perhaps roll once again in case you think you can get something better but do not want to lose your current roll, or simply to safeguard it in case of data or device loss.

Navigate to the option as indicated in the screenshot, and the game will assign an ID for you account while allowing you to create a password for it. Do note that that switching between Android and iOS will wipe your gems, so be sure not to buy any just yet or retrieve them from your inbox until you are absolutely sure.