Sapere followed by an infinitive means to know how . So use mettersi for to wear or put on clothing. Reflexive verbs will be in a later lesson. You have to include the mi, ti, si, etc. because they are parts of the conjugated verb. Mi metto la maglia is I'm wearing my sweater.
Some adjectives have two forms, others have four.
Only a few adjectives go before the noun, the rest are placed right after it. Bello and buono have alternate forms when they precede a noun. If they go after the noun, then they can be formed in the usual way . The above forms are only for when they go before the noun. Be aware that grande can have alternate forms before nouns too.