I've worked as an RA for a few professors before and it really isn't that scary. You're right that tasks vary a lot depending on the professor though, but most of the stuff I've ended up doing has not been that hard. Things I have done as an RA are literature reviews, proofreading and commenting on manuscripts, coding/cleaning data, running some analyses, making graphs and tables for papers, looking up references, formatting the reference list of a paper etc. Other people I know (we are all in psychology) have helped with running experiments. Sometimes you do get lucky and the professor lets you have more input on the project and you will get to be a co-author but this varies a lot and often depends on what stage of the project you come in on, (i.e. if you are just doing things to tidy it up at the end then it will be unlikey you will be a co-author but if you are heavily involved from the beginning then you might have a chance in being co-author).