Cooking steak on the grill First and foremost, I am a member of a few other Stack Exchange sites, and I haven't been this excited about one for a while. I'm basically a nerd that's trying to get into cooking, and I couldn't be more happy they made this :)

Onto the question,

Growing up, I've always grilled on gas grills. Recently, I've moved into an apartment with my girlfriend and we are only allowed to have a charcoal grill. It's a tiny one (I think), probably about 1.5 - 2 feet in diameter. I use the charcoal that are already covered in flammable material (the black ones), and they work pretty well. I basically follow the directions every single time I go to light the grill. I put about 30-40 bricks in the middle of the grill in a pyramid shape, light them in several places, and then watch it burn. I do this until the flame pretty much dies down and the bricks are about 80-85% covered in grey ash. After that, I put the cage back on the grill, let it heat up so I can clean it off with the brush, and then shake the bricks until they are even across the bottom.

Right after this I put the steak on (right in the middle where there's the most heat from all the bricks burning in the pyramid towards the top before I spread them out). Now at this point, I NEVER have any idea how long it should stay there, when I should flip it, if I should flip it more than once, what I should look for before I flip it, how fast it's cooking - I basically don't know anything. I'm a guy, I hate how I can't grill great! :(

I usually end up either over cooking it so it's tough, or under cooking it and it's still bleeding in the middle. I like my steaks right around medium, and I can never seem to get them perfect. I had a perfect one once, and it was 100% luck. I don't have a thermometer or anything to test the meat with (maybe I need to invest in one?), nor do I have anything that I can test how hot the grill is inside. The grill has a basic black cover with a little vent so I can control the air getting inside, but no thermometer or anything.

Can I get some tips? I'm cooking a steak on the grill for my girlfriend tonight that has been marinating for over 24 hours, and I don't want to mess it up!

P.S. And I almost forgot! The steak I am cooking tonight is a London Broil.

Thanks!
Here's a couple of hints:


Learn to judge doneness by feel. See this answer for a good guide to temp by feel. 
Learn to judge grill temp by hand. Hold your hand palm down about 3-4 inches above the grill. If you can hold it there for a second or two, it's hot; 3-4 seconds is medium; 5-6 seconds is low. 
Sear first. Start with a very hot grill. You want about two minutes per side. Do not move the steak while it is searing. Make sure you've given the grill grate at least 5 mins to heat up. 
Use a two level fire. Once you've seared your steak, you want to finish cooking over a lower heat. Build a two level fire by putting the coals on one side of the grill rather than the whole thing. Then sear over the coals. Then move it to the other side to finish. If the grill isn't wide enough for a two level fire, you could try quenching the fire by cutting off air flow. That will cool it quickly. 
Finish with the lid on. This helps cook by heating the air rather than just using direct heat. 
Change your fuel. Stop using lighter fluid. Use a chimney instead. Hard lump charcoal is better than briquettes. It's easier to light, creates less left over, and burns hotter. 
Rest your meat for a few minutes before eating. Be aware that it will continue to cook during this time, so you want to pull it off a little early. 
Read up on cooking steak. There are a bunch of good questions on this site!