1) If you're following the Mr. Beer instructions to "fill the LBK to the X mark with cold tap water", don't. Put a gallon of spring water* in the fridge overnight and use that. See my rantings and ravings on this page about the instructions being inaccurate as to why.
*Or tap water, if it isn't highly chlorinated and tastes good to you, or filtered water if you have a filter; basically if you don't like the taste of the water you put in you won't like the taste of the beer you get out.
2) Get one of the Mr. Beer stick-on thermometers and put it on your LBK. This way, you can be sure that your wort is in the proper temperature range before you pitch the yeast. Despite what some people claim, if you follow the instructions your wort is NOT always at optimum temperature to pitch the yeast. Why is that? Click on that link in point #1 above. I try and pitch the yeast between 65-68F. This sometimes necessitates me chilling the wort in a cooler with a couple of liter bottles of ice to get it down to those temps.
3) When you get to the part of the instructions that say "Put your keg in a location with a consistent temperature between 65° and 76° F (20°-25° C)", ignore that, too. It's not the temperature of the location that's important, it's the temperature of the wort in the LBK. This is also why having a stick-on thermometer helps. For most recipes, the consensus is that you want to keep the wort around 65F, at least for the first week or so of fermentation. There are exceptions, of course, but for a newer brewer doing basic recipes, keeping it around 65F should be the goal.
4) Most beers will benefit from longer conditioning times. IPAs and Weizens are exceptions. A good rule of thumb is the higher the ABV, the longer the conditioning.
5) Pure extract brews have a bit of "twang" to them. Longer conditioning helps eliminate that. Once you get more experienced and take the step up to partial mash brews you'll notice that the "twang" is less present or eliminated entirely.
6) Find one of RickBeer's posts and click on the links in his signature block. Read, read, and read.
7) Don't get discouraged. I started brewing a few years back and while the beer I made was drinkable, it was not very good. In retrospect I now know why: I was using cold tap water which was not cold enough, pitching yeast too warm, not keeping the LBK at the proper temperature, not letting it ferment long enough, and not letting it condition long enough. After a few batches I stopped brewing. I came back to it a couple of years later and Mr. Beer's products have come a long way...as has the knowledge base as to how to make a quality product.