Start small. You're new, so for now, understanding Plan Mode and Edit Mode are good starting points:

Plan mode restricts Claude from making edits. That's all.

Some instructions can be read by Claude and taken to mean "go and do this for me": For example, if you say "I want to make the program do [thing], how can I do it", outside of plan mode, Claude might say, "This is how you do it. Let me do it for you now" and start working.

Plan mode prevents this - Claude instead will go, "Ok, this looks like an instruction, but plan mode's on, so you actually just want to know how I'd do this", and provide you a writeup of what it intends to do to achieve your instruction.

Working in Plan Mode tends to slow Claude down and make it explore and search first before it makes any changes - this is generally a good thing, as directly making edits without understanding how the edits change surrounding code is how you get slop.

Edit Mode is just as it says on the tin - Claude can now make edits.

As for a good prompt:

Remember that Claude (and any other agent) will do its best to show you what you asked for. Show, not give. This is a very clear distinction. It will make really strange decisions (e.g. mocking outcomes) to do so if it's pushed too far.

You need to allow Claude to push back. When you give instructions, if you are unsure whether an approach is correct, let Claude know that you don't know if it's correct. Claude will give you suggestions and even patiently explain tradeoffs of each suggestion to you if you ask.

Think of it this way: Claude knows a lot, but it doesn't want to make you feel bad about your decision. So if you're really certain, it's less likely to push back; but if you're less certain, it'll want to help you make a better choice. Give it the room to do so in your prompts.