I highly recommend Photoshop, it’s so versatile and it’s pretty easy to learn the basics of it just by watching a good tutorial. And those basics cover everything that you need for a sell sheet and even more. The downside is the subscription but you should be able to find it at a library or reach out to your contacts and ask if they can help give you access to theirs. It’s very popular so I’m sure you won’t have trouble finding a way to do it. Good luck!