No, adverbs like "vraiment", "tellement" etc are invariable, which means they have no gender and no singular/plural. Adjectives and nouns do take an "s" (contrary to English, were adjectives are invariable).
For your second question, I guess you are asking about nouns: answer, you cannot. You have to learn each noun in its singular form with the article and remember it by heart.
I know that some grammars dedicated to foreign learners sometimes list some rules towards the gender of nouns, but it rarely works.
I think that genders in french are the hardest thing for a learner. Many natives are not sure about the gender of some words like "armistice", "pétale", "après-midi (which can be both masculine and feminine)".
We also have some words that are masculine in singular and feminine in plural, but it is extremely rare. "Un amour enivrant", but "Des amours enivrantes"... Weird.