Unless you are emetophobic, in which case I understand this might not be an option for you, eating when you feel nauseous is a good technique for reducing nausea; you're partly feeling ill because your blood sugar is down, so getting anything to stay in for a brief period is better than nothing as you'll get at least a tiny amount of energy from it - and, gross though it sounds, actually throwing up can give you a brief respite from the feeling as well.
I had HG each time and have had to take medication that causes long term nausea before now, combined with a very sore mouth, ulcers and a distorted sense of taste. Salty things, sweet things - never milky, fatty things - help deal with low blood sugar and dehydration. Plain water is likely to make you feel terrible, but small sips of Lucozade aren't as bad. If all you can manage is a packet of Pom Bears over the course of about three hours, two Haribos and some sugary fluids, that's what you do. Others can manage olives, cubes of cheese and cooked ham or suchlike. Again, it's what you can get down you and get to stay down for more than a couple of minutes that matters, whatever it is.
At the same time, be careful with brushing your teeth. You'll still need to do it, but I'd strongly recommend that you don't try and brush your tongue for a few weeks.
Outside of HG, most people seem to feel it's at its worst around 9 weeks and then it begins to ease slowly. But if you can't even do sips of drink, see the GP, as it is something that can put you into hospital if left untreated (and affects the likelihood of keeping folic acid or iron tablets down if you turn out to be anaemic).