I am not admitted to practice in Texas so I don't know how long you have to begin a probate proceeding. But you should get started immediately. You need to make an appointment with a local probate lawyer. Ask for a complimentary consultation. And also ask how they charge for their probate services (and when the payment is due - in California we are not allowed to collect any payments for our services until there is a court order distributing the probate property to the heirs).
The information you obtain at this site is not, nor is it intended to be, legal advice. You should consult an attorney for individual advice regarding your own situation.
I agree that you need to contact a probate lawyer in your area as soon as possible, so that your dad's property doesn't disappear.
If you don't know a lawyer, contact the city/county or state bar association, or the local chamber of commerce for a referral.
Here's a link to a site with some basic information about intestate succession in Texas. (Intestate succession means who inherits if there is no will.)