How do I say 'I want this'?

要 is the direct “want” — it gets the job done but can land bluntly without context. The art of ordering in Chinese is about calibrating directness:

• 我要这个 (pointing) — standard and fine. The pointing + 这个 softens it by making it about the specific item, not a demand.
• 我想点… — “I’d like to order…” Adds the thinking/considering layer (想), making it more polite.
• 来一个 — the efficient insider form. “Bring one.” Common at street stalls, casual restaurants, and among people who eat out regularly. Slightly northern in feel but understood everywhere.

The cultural context: Chinese restaurant ordering is generally more direct than English. Wait staff expect you to state what you want without elaborate politeness. 我要 is not rude in context — it is efficient. But in a high-end restaurant or when you want to be extra courteous, 我想点… is the safer choice.