Adjectives change their endings to agree in gender, number and case with the nouns they describe
Shorter adjectives add "-ši". For example lep (pretty) becomes lepši (prettier). Longer adjectives add "-ejši". For example star (old) becomes starejši (older). Sometimes "-ji" is added as with drag (expensive) which becomes dražji (more expensive)
Bolj (more) and manj (less) can also be used with the base adjective: bolj inteligentna and manj znan (less famous)
The genitive case with the preposition, od (than), can be used: Večji sem od tebe (I am taller than you).
The nominative case with the preposition, kot (than), can also be used: Večji sem kot ti (I am taller than you).
A dictionary is likely to show the root form of the adjective plus the masculine, feminine and neuter forms.
For example in the PONS online dictionary the English word "calendar" translates to the noun, koledar and also to the adjective koledarski. Double clicking on the latter gives the following: koledarsk|i <-a, -o> ADJ. This indicates that the root of the word is koledarsk; the masculine form is koledarski, the feminine form is koledarska and the neuter form is koledarsko.
Note that Slovene adjectives not only decline to agree with the gender as shown above but also the number and case of the noun they describe.