I'm having thick yellowish mucus, antihistamines doesn't help, what else can I do?

Salt water. I always recommend that people try some form of sinus irrigation for such symptoms. There are different varieties of sinus flushes that you can find over the counter. These work quite well in clearing the mucus and even helping with congestion. If it continues I would recommend seeing your primary md or even an ent.
Depends. If it's been going on for less than a week I recommend hydration, mucinex, (guaifenesin) saline spray. If longer and you are not noticing improvement in symptoms you may need an antibiotic. Contact your primary care md or an ENT doctor if necessary.