Paste deletes last character?

When I copy/paste, the previous character(s), whether a space, symbol, or letter is deleted. It will highlight and even if you click away to try to remove the highlight, it is not possible. When pasting - it will delete whatever was highlighted in that area.

I also received (urgh) a PC keyboard/mouse with my computer, so shortcuts are not possible right now - or I would try a shortcut to see if it helped. But, if I remember correctly, even shortcuts on my old Macbook didn't seem to help - or I was doing so incorrectly.

Can someone help me with this issue, please? I cannot seem to find anything about it on google or any other Mac reference. Perhaps it is my wording - and I apologize. But I truly would appreciate the assistance.

Thanks in advance!

-Hav