Will I go to jail for a first-time DUI?

Possibly, though for a straightforward first offense it isn't the usual outcome. Judges can order probation in lieu of some or all jail time, and on a first conviction with no aggravating circumstances that's common.

What shifts it toward actual jail: a reading at or above .15, a minor in the vehicle, a crash, property damage, injury, or conduct during the arrest that ends up in the report.

What shifts it away: no prior record, early completion of DUI school, treatment already underway, and a case where the State's evidence has problems worth negotiating around.

Note that a second offense within five years carries a mandatory minimum of ten days, and a third within ten years carries a mandatory thirty. Those aren't discretionary. The first offense is where discretion still exists, which is one reason the first case is worth defending properly rather than disposing of quickly.