Many students worry too much about grammar. For speaking parts 2 and 3, it's more important to worry about the quality of your answers. The best way to improve the quality of your answers is by adding detail.
Describe your best friend. Say when you met him/her.
Short answer about when we met:
I met my best friend at school when I was 11 years old.
Detailed answer about when we met:
I met my best friend at school when I was 11 years old, so we've known each other for ... years. I remember we sat next to each other in my first science lesson at secondary school, and we had to work together to do an experiment. We got on straight away, and we've been friends ever since.
Notice that by adding more information I've also used more/better vocabulary and grammar.