L'étagère est haute ; je la trouve pratique pour ranger le sucre et le thé.

Because étagère begins with a vowel sound. In French, the feminine singular article la elides before a vowel or mute h:
la + étagère → l’étagère

Adjectives in French must agree in gender and number with the noun they modify. Étagère is feminine singular, so:
haut (masculine singular) → haute (feminine singular)

A semicolon links two independent but related clauses more strongly than a period and more cleanly than a comma.
• With a comma you’d risk a run-on sentence.
• A period would be correct but separates the ideas more forcefully.
Using ; shows “the shelf is tall” and “I find it practical…” are distinct yet closely connected thoughts.

je la trouve literally means “I find it.”
• je = “I” (subject)
• la = direct-object pronoun replacing l’étagère (feminine singular)
• trouve = first-person singular of trouver (“to find”)
Word order in French is subject – pronoun – verb: “Je la trouve.”

pratique is an adjective that happens to end in -e in its base form. For singular usage, both masculine and feminine forms are identical:
masculine singular pratique = feminine singular pratique
Only the plural adds an -s (pratiques).

After pour expressing purpose (“in order to”), French uses the infinitive form of the verb without any additional preposition. Structure:
pour + infinitive = “in order to ”
So pour ranger = “in order to store/put away.”