I would recommend planning to arrive at the LSAT testing center about an hour early. This will allow you plenty of extra time to allow for delays in your commute (e.g., slow traffic, bad weather, car trouble, etc.).
Of course… you may want to adjust this based on how far you will be driving and how you will be commuting. If you live 5 minutes from the testing center, you might only want to plan on arriving 30 minutes early. On the other hand, if you live 3 hours away from the testing center, you may want to plan on arriving 90 minutes early in order to account for potential delays in your commute.
The last thing that you want to do is arrive late which may prevent you from taking the LSAT or, at a minimum, cause unnecessary stress that could have a negative impact on your performance. It is better to be safe than sorry.
Each LSAT testing location will have their own policy regarding when “doors open” and students can begin entering the building. Your best bet would be to call the actual testing center location and ask what time the building opens on the day of your LSAT administration.
Even better, I would recommend visiting the testing center. Preferably at the same time of day and day of week that you will be taking the exam. Time your commute so that you have a realistic idea of how long you can expect it to take you to get there. Check out the parking situation at your LSAT testing center in order to plan around any potential difficulties that you may encounter parking on test day.
Go inside the testing center and find someone who might be able to direct you to the room where the LSAT will be administered. Ask any questions that you might have (e.g., what time does the building open? where is the best place for visitors to park?). Make note of anything else that might have otherwise surprised you on test day (e.g., is the temperature in the building especially hot/cold? is it noisy? are the desks in the exam room small?).
The point of this visit is to eliminate, to the extent possible, any surprises on the day you take the LSAT and to answer the type of questions that you have asked here.
If physically visiting the location isn’t possible, you should at least call the testing center and ask these same kinds of questions. Getting these sources of anxiety resolved prior to exam day will go a long way toward ensuring that you are in the right state of mind when you finally sit down to take the LSAT.