Ok, the big assumption most shop keeps make is that anyone that's "into" magic will recognize the names of those doing a lecture either because of reading about them in a magazine or by way of a club such as the IBM or SAM, etc.
If the shop keep knows you and your background he may be able to suggest which lectures would be best for you based on your present skill level and willingness to do the work... at least that's how it was back in the good ole days, I've been a bit disappointed by shops over the past ten years or so, when it comes to their actually helping the newbie.
Most lectures are going to deal with close-up type bits with Card Tricks topping the list... in fact most magic clubs will not book a speaker for a return lecture if they don't teach at least one or two "good" card tricks... and I so wish that was must my exaggerating but I've seen them get mad at Eugene Burger for not doing any card tricks in his talks.
As you move deeper into things you will get to where you kind of know the who's who in your region/country and thus, you would have a better idea as to if or not a lecture would be of benefit to you. HOWEVER, if and when you get the chance to see one of the old timers that seems to have one foot in the grave DO SO! You never know when they won't be around any longer and you will kick yourself for the rest of your life for missing out on such opportunities. I know I do and I've seen many of the greats over the years but likewise, missed out on a few, like John Calvert
Don't over think it all, you'll find it all smoothing itself with time. But you may want to subscribe to one or two of the big Magic Magazines like MAGIC or Genii or simply catch the on line monthlies like VISIONS