Like Aelwyn cast a circle of salt if you're worried about negative entities. I wouldn't ask a spirit to reveal itself to me and I've heard it's not the best thing to do. I have a homemade wooden spirit board which I've only tried to use once, with no results.
Remember Good and Bad are subjective things. What the spirit sees as good, could be what you see as evil. Remember that spirits can lie to you, so you cannot always believe what they say.
If you just want to contact spirit guides simply go on a Shamanic Journey. Never ask questions invovling you or anyone elses deaths. For example don't ask, "When will Sarah die?". I've also heard that spirit boards can be addicting and that you should refrain from using the board more than twice a week.
It's a good idea to have another person with you. Make sure they have a notebook and something to write with because sometimes it can move fast.
If you are going to make your own board get wood(or paper but wood makes it so it is reuseable) write numbers 0-9 on it letters a-z, yes, no, and Goodbye. You may add other things as well.
Never end a session without thanking the spirit and moving the pendulum or whatever you're using to Goodbye.