Start with safety. If a tree is on the roof, power lines are down, or a ceiling is sagging with water, keep everyone out of that part of the house and call the utility company before anything else. Nothing on the roof is worth a fall or a shock.
Next, photograph everything before it's touched: the roof from the ground, ceilings, walls, floors, and any damaged belongings. Take wide shots and close-ups. Dated photos taken before any cleanup are the strongest evidence your insurance claim can have.
Then stop the water. Most policies expect you to take reasonable steps to prevent further damage, and a tarp over an open roof is the most common one. Keep the receipts for tarping and emergency work, because they are often reimbursable.
Call your insurance company to open a claim, and write down the claim number and your adjuster's name. Then get a written estimate from a contractor who will actually do the repair. A detailed line-item estimate gives your adjuster something concrete to review.
Finally, be wary of anyone knocking on doors after a storm and promising to cover your deductible. In Texas, contractors are not allowed to waive or absorb a deductible. A contractor who offers to is a red flag, and the offer can put your claim at risk.