я буду делать - I will be doing - it's a process in the future я сделаю - I will do \ I will have done - a process will be over

я буду делать - I am going to do something (not 100% result) я сделаю - I will do something (definitely 100% result)

"я сделаю" means not only "I will do", but also "I'll have it done". You view an action as complete by some moment in the future (perfective aspect). The moment is not necessarily indicated, but the resolution is stated.