http://www.erowid.org/chemicals/dxm/dxm.shtml
Read the vault, especially the FAQ. DXM is a relatively safe drug but it does have a number of drug interactions and you need to make sure that you get the right product.
The best product to get will have the only active ingredient as dextromethorphan HBr. The only other active ingredient that is okay (but can give you some pretty brutal diarrhea in high doses) is guaifenisin. Robitussin DM contains guaifenisin and dextromethorphan. From what I heard, Robitussin pulled the regular Robitussin Maximum Strength Cough off of the market which was the easiest to go down (it was an alcohol/dextromethorphan mix) - don't get any product that says cough and cold or anything similar as these usually have pseudoephedrine or antihistamines which can be fatal in overdose.
Do a lot of research on it before you do it. It is a very intense experience if you aren't properly prepared for it (and quite often even with proper preparation it is still overly intense).
If you take any prescription meds, check for interactions with dextromethorphan. Avoid DXM if you are on anti-psychotics or anti-depressants and never mix it with ecstasy.
Always check the active ingredients because the wrong active ingredients can kill you. This is the number one rule of DXM.