As would be expected in a state as big as Texas, climate varies, sometimes dramatically, by location; it can be snowing in one area of the state, such as Amarillo, while people are swimming at South Padre Island. High temperatures in the summer average in the 90s (30s Celsius) in most of the state, while average winter temperatures drop -- sometimes much lower than you might expect -- as you travel north. Southern Texas is known for its muggy summers, which make it feel hotter than it really is, and contrasts with the dryness of the West Texas deserts. The state's few mountainous areas have more extremes of temperatures, hitting the 80s and 90s (upper 20s and 30s Celsius) during the day, only to plummet into the 30s and 40s (single digits Celsius) at night. All areas of Texas get more sunshine than most other parts of the United States.
The beaches along the Gulf Coast are busiest in winter, although they're seldom really crowded. But unless you're a college kid looking for some rowdy spring-break action, you should avoid all resort areas, including the beaches and national parks, during March and early April.
