Common adjectives such as big, small, new or old will go before the noun, but usually adjectives will go after the noun in French.
e.g. C’est un grand bâtiment
C’est un train rapide
All adjectives must "agree" with the noun they describe – if the noun is feminine and singular, the adjective must also be feminine and singular, or if the noun is plural, the adjective must be plural.
In a dictionary you will find the masculine singular form. Regular adjectives are made feminine by adding an "e", and plural by adding an "s".
e.g. masc/sing fem/sing masc/pl fem/pl
grand grande grands grandes
petit petite petits petites
Irregular adjectives must be learned separately
e.g. masc/sing masc/sing(vowel/h) fem/sing masc/pl fem/pl
nouveau nouvel nouvelle nouveaux nouvelles
vieux vieil vieille vieux vieilles
beau bel belle beaux belles
gentil - gentille gentils gentilles