1) Use a comma to separate items on a list. This always reminds me of a memory game I used to play with my kids on long car journeys:

I went to the shops and I bought an apple, a banana, a cherry and six bars of chocolate.

Be careful to avoid confusion here though:

I went to the shops and bought my favourite sandwiches – hummus, sardine and cheese and tomato. 

Now, are we suggesting here that I eat sandwiches with all these fillings? Or that I like hummus sandwiches, and sandwiches that contain sardines AND cheese AND tomato. Or sardine and cheese. Or just cheese and tomato? Commas can clear this up:

I went to the shops and bought my favourite sandwiches – hummus, sardine, and cheese and tomato.

2) Use a comma to separate a series of actions, events or elements in a sentence:

She opened the…