I am assuming you purchased the Canon mount version of the lens. It is an EF mount, not EF-S. Make sure you are lining up the mark on the lens with red dot on camera, not the white square.
Sigma makes lenses in many different camera mounts. You need one with the Canon EF mount for it to fit onto and work with your camera. I don't know why Trevor assumed it's a Sony mount lens. The information you've provided doesn't give us any idea what it's intended to fit. But since it doesn't fit your 1300D, it must be something other than the Canon EF mount version. Besides those made for Canon EF mount, the now-discontinued Sigma 70-300mm f/4-5.6 DG Macro lens also was made to fit Nikon, Sony, Pentax, and Sigma's own cameras. None of these other four will are compatible with your camera.
There is probably a code somewhere on the lens indicating what it fits... or we might be able to tell by looking at a photo of the lens' bayonet mount. Regardless, you probably will need to return it. If the seller doesn't have a Sigma 70-300mm to fit your camera, there have been a series of them made by Canon that would be sure to fit and work on your camera.
There also have been some 70-300mm lenses made by Tamron. Like the Sigma, the Tamron also come in various mounts, including Canon EF. But there are so many options just among Canon's own lenses, there's little reason to go to third party like Tamron and Sigma.
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My first response, now deleted, was based on me reading your message as saying Sony, not Sigma! 😛 Early in the morning and no coffee yet! Sorry about that!
If you have the Sigma EF mount lens, you will notice that there is a red and white dot on the lens in line with each other. You need to line either of those dots up with the Red dot on your camera body.
Alan has given a masterly summation of your offerings if you decide to go to a Canon lens brand, and you can get the EF 70-300 models via the canon Refurb store, price reduced, as new with a one-year warranty. I enclose a link to keep an eye on the site for when they come available HERE
Definitely stay away from the 75-300 series, they are pretty poor optically.
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