You can fill the bags with rock or sand. The weight is what would be manageable for you but not to exceed three quarters of the bags capacity. This is so you can tie the top. 

Sandbag weight averages 10-15 pounds (4.5-6.8 kg) depending on amount and type of fill material. 

How much sand will you need? On average, one cubic yard will fill approximately 180-270 bags.