1. Redox reactions

2. First things to do with your reaction
Firstly balance your equation
Next:
Calculate the oxidation number of each atom.
Remember, compounds have an overall oxidation state of 0 (unless otherwise specified)
In compounds H=+1 and O is usually -2

3. How to identify the oxidised atom
OILRIG
Oxidation is losing electrons
So the atom which has an oxidation number that is increasing is the oxidised atom

4. How to identify the Reduced atom
OILRIG
Reduction is gaining electrons
So the atom which has an oxidation number that is decreasing is the reduced atom

5. Example Ca (s) + H2SO4 (aq) CaSO4(aq) + H2 (g) So
So Calcium therefore Ca is oxidised
Hydrogen therefore H is reduced

6. How to identify the oxidising agent
Now you have identified what is being oxidised and what is being reduced.
An oxidising agent is the part of the equation that causes oxidation by taking electrons from something else
Therefore the compound that contains the object that is reduced is the oxidising agent!
In the case of our example it is sulphuric acid (acids are often the oxidising agent because they contain H+ ions)

7. Oxidising agent reduced = OAR

8. How to identify the reducing agent
Now you have identified what is being oxidised and what is being reduced.
An reducing agent is the part of the equation that causes reduction by giving electrons to something else
Therefore the compound that contains the object that is oxidised is the reducing agent!
In the case of our example it is calcium