Kill power to the affected area if water is near outlets, photograph everything before you touch it, then get the water out and air moving. Document first, because the photographs are what an insurance claim and any future disclosure rest on.
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The first hour is about safety and evidence, in that order. Safety. If water has reached outlets, a furnace, a water heater, or an electrical panel, shut off power to that circuit or the whole basement at the panel before stepping in. Standing water plus energized equipment is the one genuine hazard in an otherwise unpleasant situation. Gas appliances that have been submerged should not be relit without a technician. Evidence. Photograph before moving anything: the depth against a wall, where water is entering if you can see it, wet boxes and furniture in place, and the exterior conditions including downspouts and grading. Take more than feels necessary. If there is a claim, or a disclosure form to complete at sale, or a contractor arguing about cause later, this is the record. Note the time and what the weather was doing. Then extraction and drying. Get standing water out, remove wet porous material that cannot be dried within a couple of days, and run air movement and dehumidification continuously. Carpet padding, cardboard, and wet drywall below the water line are usually losses. One diagnostic worth capturing while it is happening: where the water is actually coming in. Water at the joint where the wall meets the floor is hydrostatic pressure pushing up through the cove joint, which is the most common pattern in this market. Water running down a wall from higher up points at a window well, a wall penetration, or grading. Water through a visible crack is its own case. A first-time leak in a basement that has been dry for years almost always means something outside changed: a downspout that came apart, grading that settled, a neighbor regrading, or a storm that simply exceeded what the site could handle.