Make sure your friend is okay when you sense something is wrong.
Know when to be serious, and know when to be goofy.
Go the extra mile when they ask for help.
Don't give up on them during their darkest hours.
Make them feel wanted.
Just be yourself around the person you want to care about, and try your hardest.
There's no really BIG critical requirements for a good friendship, you just know what's right from wrong.
Hope I helped.