As far as opiates and opioids go, codeine is probably the weakest of all to try for. Sometimes the effects are rather subtle and it can take a few separate doses to fully recognise them. I do recommend it for beginners though and even for experienced users it can be fun. It is most commonly found in tablets or capsules in combination with aspirin or paracetemol. Usually these tablets have Paracetamol 500mg / Codeine 8 - 9.75mg. The paracetamol should be extracted (link to procedure below) as the amount ingested without extraction would be harmful to the liver.
Dosages
Light high - 100 - 200mg
Medium Intensity high - 200 - 300mg
Heavy high - 300 - 550mg
Note: I have neither experienced or recieved word of dosages exceeding 550mg so be careful if you plan on doing so.
Codeine does make many people nauseous but this feeling will generally pass when the hit fully kicks in. Many people are also not satisfied with codeine because like I said before the effects are rather subtle the first few times. If you really want to feel it, try taking it on atleast three separate occasions.
Click here for a procedure of extraction (from erowid.org). The procedure is located about halfway down the linked page.