I'm sure I've posted about this before, but it always gets lost (and it's not like I'm an authority on the matter anyway), but...
This whole "protection from evil" thing is a fairly common question. The problem is that it's not certain, not only if Astral Projection takes place as many people think it does, but if these "other entities" are present at all anyway.
Read a work like Sylvan Muldoon's "Projection of the Astral Body", and he speaks not of any other entities being present in any conscious astral projections. Go and read Robert Bruce's "Astral Dynamics", and he does. Different authors have different opinions, which, if their claims to astral project are legitimate, bring up some very interesting questions.
In any case, if you're worried about "evil" doing anything to you during projection, different people have different ideas.
If you're religious, I've seen some people put a strong religious symbol on their bed to protect them and ward off evil. Others use the old "line of salt around the bed" trick. Most tend to do some form of affirmations during meditation, or chakra and kundalini work if you believe in that. Honestly, things like Astral Projection shouldn't really be rushed anyway; they take time and patience. In the meantime, I encourage you to get your hands on as many resources as you can on Astral Projection. The two books I named are good; for more "online based" resources, there are various Youtube videos about it, and people like Steve Jones speaks a lot of it (but then, he seems to cover everything...)
So, yeah; engross yourself in learning, and please, keep asking as many questions as you require. :-P