You definitely need to be straight with boy 'A' as it is not fair to have him think he may have a chance with you, when at heart you just like him as a friend. Just tell him you love him as a friend but nothing more, yes he will be upset but give him time and he will understand. It's better to tell him now rather than string him along and break his heart at the end of it. DON'T tell boy 'A' just yet that you fancy his best mate, firstly tell him asap that you just want him as a friend, and in the meantime while he is getting use to the idea, try and find out how boy 'B' feels about you. If in the future you find out that boy 'B' does like you the way you like him, then give things a go! Hopefully by this time boy 'A' will have got use to the idea of just having you as a friend, and should understand and support you and boy 'B'. Hope this helps!