People give you karma by clicking on the "Karma" button at the top-right of a post. You can adjust the amount of karma to give, but I suspect most people simply give the default amount. As far as I can tell, the default amount is a percentage of what the giver has. Initially, you'll only be able to give others a few points of karma but if someone with lots of karma gives you karma, you'll suddenly have a lot, and you'll be able to give others a lot more.
This is the FAQ entry. It doesn't say how the amount you can give is worked out, though:
https://www.mobileread.com/forums/faq...vb3_reputation