I'm brand new to this and any help/suggestions I can get would be appreciated. I googled this stamp, and they all look like they're very valuable. I think it's an A140, but like I said I'm brand new to this.
I found it in a collection I bought today. I found several stamps like this on google that had fairly high prices. I also posted this on r/philately and someone told me that there are many like this, but only a handful of valuable ones.
It's a common stamp that will not sell for any amount of money. That design has dozens of varieties of major catalog numbers. The expensive ones are rare, but you have one of the ones that was printed in the tens of billions of copies. Someone will post a stampsmarter link. You can start there to learn about the different varieties and check to see if you won the lottery when you find stamps that look like this one. You have a sheet stamp, not a coil or booklet stamp which rules out a lot of varieties, including the most expensive ones. Next to check is the perforations. You need a perforation gauge to do it. You can print one or buy one. You don't count perforations or use a ruler. Then you will need to check watermarks. If you are interested in stamps, get a Scott specialized catalog for US Stamps. Beyond the online references, the Scott catalog is where to start.