What are the rules for buying and selling property for profit?

I want to buy things, and sell them for profit. Just normal property at first. Not big ticket items like cars, houses, or boats (nothing with titles or that I’d need to be a licensed dealer to negotiate on a regular business.)

What do I need to do to protect myself tax-wise, and what are some good practices?

I’ve been told to keep my receipts, and the location where I bought whatever I buy. I’ve also been told to just keep a log of the transactions, so that at the end of the year I can do my taxes accordingly.

I guess I am confused as to some of the technical issues with the money. If I start with $20 and buy something that I sell for $40, does it matter if I take the taxes out right then or should I wait till the end of the year to do a lump sum capital gain form?