I'm sure everyone knows this already but it's my favourite so I'll post it anyway,
Tomatoes ( I always use the big ones but it is just personal taste.)
Basically cut up the tomatoes into small pieces and then dice the onion and chop coriander. Mix them in a bowl with the other ingredients and a bit of olive oil. Sometimes I use lemon juice instead of lime or add a bit of crushed garlic.
No real quantities of each it depends on personal taste, I always add loads of coriander but that might not be to everyones taste.
Also it can be fairly watery but if you don't use the tomato pulp it is a bit "drier"