Don't be scared. North America is a safe destination.
You will be fine with a suitcase, as long as you aren't planning on doing any hiking and camping. I have never had a problem leaving my suitcase in hotels in the U.S. I haven't stayed at many hostels, but assuming you bring a lock you should be fine. Take the usual precautions - don't leave money or valuables lying around in plain sight.
As for information about America, the best thing to do is to go to the local tourist center or city hall when you arrive at a destination. They will usually have a map of the area listing the sites. Larger destinations (and even some smaller ones) will have web sites. Do a search on google before you get someplace.
There really is no reason to be scared in either the US or Canada. Generally, you are safer here than in most of the world. I travel extensively for business and have never had anything taken from a hotel room. You have to use common sense and not leave cash or expensive items out in the open, but as long as you use your head you should be fine.
Some big cities are dangerous if you go into the wrong neighborhoods, but there usually aren't tourist type destinations in those areas. Quite often the tourist area is fine, but away from the tourist area is not. For example, the inner harbor in Baltimore is great, but you stay near the water as several blocks inland is a pretty rough neighborhood. Again, you have to use your head.
Enjoy, the US and Canada are great places to visit and see the sights. Most of the people you meet will be helpful and friendly.
North America is very friendly. There is nothing to be scared of. In most cities along your trip, you can just ask anyone one the street for things to do. I've found the best way to see cool, little-known sites is to ask a bartender. They will usually hook you up with the fun spots and cool stuff. Hotels are great, as long as there is no cash laying aound on a table or something, you should be fine. Have fun.
thanks everyone for all your advice, it's a great relief, i've never travelled as an individual before!!! thanks heaps!