If you have not already done so, download Avogadro for free and watch the tutorials if necessary.
Build the n-heptane molecule, by checking the properties of the molecule (sequence Wiew → Properties) decide if you have built the right structure.
Just by moving the C-atoms, build all the isomers of the compound. Watch the tutorial!
You can check if the new molecule you have built is an isomer of the n-pentane by comparing the molecular formula and the molecular masses.