Different types of strings can be used to create different sounds and feels. Round wound strings are the most popular strings for modern bass playing. The gauge varies between light, medium and heavy. A standard medium gauge would probably be the best to start with e.g., G = 45, D = 60, A = 80, E = 105. The heavier or thicker the gauge the "bigger" the sound will be. The lighter the strings the easier and faster you can play.
Always clean your strings with a cloth after playing as this will keep the sound brighter. To get an optimal sound, and if you can afford it, change your strings several times a year, especially before an important session or recording.