The first decision to make is the type of fuel the grill will use: electric, gas, charcoal or hardwood (pellets). There are firm battle lines drawn as to the best fuel, but really, it is a matter of tradeoffs. The more tasty the grilling, the more inconvenient or expensive the grilling. Hardwood (or rather pellets) provides a strong, smoky, fresh from the fire taste but the pellet grills are the most expensive and the pellet fuel can be difficult to find. Electric is the tops in convenience but leaves most people dissatisfied with the flavor. That leaves charcoal and gas duking it out in the flavor category. Charcoal may be preferred with steaks but most people cannot tell the difference between gas and charcoal when it comes to hamburgers and other meats and fishes.
Consider where you will be flipping your burgers before making a decision. If the location is on a small patio or covered porch, a large charcoal grill is not practical. Keep in mind you will have to dispose of the remaining charcoal... they stay hot for a very long time. Gas grills can flare up and need to be away from the house or apartment. Electric grills are safest, but as noted, impart the least taste to your cooking.
Most grills operate on propane or natural gas. Propane requires a propane tank. If it is not included in the grill package you will need to purchase one for about $25. If you have a natural gas line available nearby, buy a grill specially adapted to that source. You may find the grill model number includes an "N" for natural gas. There are conversion kits as well but may require a professional to apply the modification.
Your lifestyle may also influence your choice. If you envision coming home, throwing the chef's apron over your head and the steaks on the grill for a quick meal, look at the gas or electric models. But if time and convenience are second to taste and perhaps price, consider hardwood or charcoal.
Make sure the grill you are considering has charcoal briquettes, lava rocks, or even metal plates or bars that maintain and distribute the heat to the food from the actual flames, or you may experience uneven and unsatisfactory cooking.