You might want to get a sitter to be on the safe side, DXM trips can range in intensity and i wouldnt do it in a place with alot of unsafe objects that u might accidently fall or trip on on end up accidently hurting yourself. Tho, most of the time on the 3rd palteu you dont really feel like moving around too much. As far as freaking out, you can have a good or bad trip with any drug, the feelings can be strange and take a little getting used to, but going into the trip with a clear mind and being in a good mood can help insure that u will have a good time.

http://www.erowid.org/chemicals/dxm/faq/dxm_faq.shtml Thats a full FAQ on DXM that should answer all your questions.

You said that your between 100 and 120 lbs, so your dosage would probably be roughly somewhere between 500-600 mg. If that isnt enough then you can increase your dosage.(the extraction method that slipknot mentionted is a better way to do it since you would be getting pure dxm and not the polisterix form) Also i forgot to mention earlier, that some people can have a serious alergic reaction to DXM. Even tho its rare, you should probably do a low plateu does first(200 mg or so) just to be sure. Wait at least 2 days between trips. Peace.