I have just started creating my own conlang, and I was wondering if anybody could help me. I can't find anything that'll help me differentiate plurality and gender.
In general, gender is an inherent property of a lexeme, while number is something that can easily be changed. For example, Latin mensae "tables (plural)" is a perfectly normal inflection of mensa "table", but *mensus "table (masculine)" is very weird.
Now, this line is sometimes blurred—gender and number interact in interesting ways in the Bantu languages, for example, where they're both part of a single marking (generally called "noun class" and sometimes incorporating other types of marking as well), and in Kiowa, each gender has an "expected" or "default" number, which you mark any deviation from. And many languages have ways to derive new nouns by changing the gender of existing ones, in the cases where it makes sense: Latin doesn't allow *mensus, but would allow, say, candidata "political candidate (feminine)" < candidatus "political candidate (masculine)". But as a general cross-linguistic rule, number can be changed more easily than gender can, and nominal lexemes tend to have an inherent gender but no inherent number.
Plural is a grammaticalized semantic concept – more than one. It might be signaled by an affix or other morphological process, or by adding a separate grammatical word indicating "plural". Gender is fundamentally about a system of morphological classification of nouns into "types". A language might for example distinguish masculine and feminine, or animate and inanimate, but the semantic bases can get rather obscure (e.g. Spanish piedra "stone" is feminine but stones are not female). There is a connection between gender and plurality (number) in that the marking of number is often one aspect of the gender system. Bantu languages have a rich gender system where nouns have a prefix (assigned a conventional number) and the choice of specific prefix depends on the "gender" (pairing of singular and plural prefixes). Thus you have pairing like 1-2, 3-4, 5-6, 7-8, 9-10, 11-10, 12-13 where the first number of the "class" prefix for the singular and the second is the plural. The assignment of a root to gender 1-2 might mean "person who X", to 7-8 it means "thing for X with", and so on. To some extent a gender might be unpredictable ("stone" in Spanish), but it might also be mutable depending on some semantic criterion.
Gender (or noun class) and Number (which includes singular, dual, paucal, and plural) are two of the possible properties of Entities, the things that Nouns refer to.
Other common noun categories include Person (first, second, third, fourth, etc.), Case (nominative, genitive, ergative, locative, etc.), Animacy (human, animal, etc.), Specificity (or Definiteness), and Honorifics (formal vs familiar forms).
And these are only the properties of nouns. Verbs have more fun (verb properties include Tense, Aspect, Mood, and Voice, plus agreement echoes of various Noun categories).