Read all of this post before you do anything option 2 might be easier.....
lm-sensors. If you're using ubuntu, goto system. then administration, synaptic package manager and search for lm-sensors. click the empty box, select mark for installation, then the big apply button. Might tell you it needs additional packages just agree usually. might have to click another "ok" button. Then after it installs open a terminal, and type sudo sensors give your pswrd and it should (if everything worked correctly a temp reading for at least the cpu.
OR!!!! right click on the top menu bar, add to panel, and look for hardware sensors monitor. that will give you readouts on the menu bar. BUT!!! I think this option will only show up after you install lm-sensors but you might give it a try before you install lm-sensors to check that.....