I panicked in my 5th month when I saw tiny lines and started applying cream immediately. But honestly, starting from the second trimester is best to keep skin elastic. It's totally normal to get them!
You can start moisturizing early, but please ask your doctor which ingredients to avoid in stretch mark creams. Some active ingredients are not recommended during pregnancy.
I started from my 4th month using basic coconut oil and cocoa butter. My stretch marks faded a lot post-delivery, so don't stress too much about them right now.
You can start around the 3rd or 4th month as your belly begins to grow. Just ensure the cream is safe and organic. Talk to your doctor to recommend a safe brand for peace of mind.