An introduction to energy changes during reactions . . .
Exothermic and endothermic reactions explained.
Exothermic reactions . . .
Examples of simple exothermic reactions.
Endothermic reactions . . .
Examples of simple endothermic reactions including thermal decomposition.
Calculations involving energy changes . . .
This link will take you to the menu of the part of the site dealing with chemistry calculations. You will need to explore the last two links on that page.
To make sense of those, you will need to be happy about moles. If you aren't, then you should read the first and second links in the list about relative atomic masses and the mole concept before you do anything else.