When God asks us to obey him in the things that he asks us to do that are difficult, it takes, as the question suggests, faith. What does that entail, though? What does having faith mean? These are tough questions that I think we all have to answer at some point or another.
The reason I think these are tough questions is not because I think they are difficult to answer, but because I think they are difficult to live out. I think the answer is simple: having faith means trusting God. In the context of responding to things he has asked us to do, I think it means being obedient and trusting that he will see us through. Doing that, however, is easier said than done. I think one way to live this out is to just take things one small step at a time, trusting in God each step of the way. I think that way we are less likely to be overwhelmed. It seems to me that having the support of other believers also helps, in addition to staying close to God daily and drawing upon his strength to live out what he has called us to do. Again, however, all this is easier said than done. Perhaps the best way to answer these questions is to go and try living out faith and obedience, realizing that we will grow as we do.