I think the key to that answer might be in "But you will receive power when the Holy Spirit has come upon you".
Just let the Holy Spirit direct you without thinking what you have to be saying and doing, and assume that what you do is what you are being guided to do.
We, Christians, are all called to witness, some of by preaching on Sunday morning and others to just give a simple blessing to someone we see as needing a kind word. (an aside here. You can cast a blessing on someone as easily as casting a curse on him, but which of these do you think happens more often?)
Remember, everything works together for good for those called to serve God;s purpose, and every Christian is called to serve God's purpose, often when we don't even know we are doing so.
We don't need to worry about how we will witness, we just need to be Christian in all things we do and say and know that that thisi s an invitation for the Holy Spirit to guide us in being a witness even if we don't know we are doing so.
This is how I've thought about it for some time now, to each his own.
