What high grade bonds are suitable for beginners?

I think it's great you are asking questions about investment.

You may be keen to know that buying bonds are not necessarily suitable for beginners any more than stocks are only for experts.

Bonds are just a low volatility debt instrument. But being a beginner doesn't mean you have low tolerance to volatility (often the case, but not always true).

Assess your risk profile and investment goals, timeline. Based on that you'll have a better idea of whether you want to invest in bonds.