Simple answer: check the prices at your silver/gold dealer. Compare the price he sells an ounce with the price where he buys an ounce. Thats his margin. Keep the margin as low as possible. And buy popular coins. Usually the margins are higher at small coins, so if you cannot afford an ounce of gold, stay with silver.
I personally stick with silver canadian maple leafs, and with gold I buy "vreneli", that´s an old swiss coin, containing 5.6 grams of gold, price is about 200$. And I don´t buy bars, because they are all numbered and can be backtracked. And finally: pay in cash, anonymously. No credit card.