[Daradon]

A simple one I like and use all the time is:

1/3 bottle beer
Generous amounts of soy sauce
Couple splashes balsamic vinegar
Several shakes of Club House Steak Spice (really just pepper, salt, dried onion etc. if you use the real ingredients cracked pepper and sea salt works much better than the ground up versions)

Volumes can change based on steak size, container size, and your own taste, but you'll obviously have to try the steak once to see what you like. Dip both sides of the steak in the mixture before you seal it up, and then flip it a few times before you grill it so both sides get some time to soak. You can do more than one at a time as well, I try not to have them on top of each other though, instead side by side, so they can both be in the marinade.

I like a stronger taste so I'll often marinate it for up to a day in the fridge, often pulling it out the day before so it can defrost naturally (skip the step if the steak is fresh obviously) and then marinate overnight. But 3-4 hours will allow enough time for the flavor to soak in and the meat to soften a little.

As others have mentioned, many salad dressings work great as marinades as do some barbeque sauces. It can be very fun to experiment. Think of the spices and flavors you like and try some of your own once you get a feel for what flavors work good together.

EDIT: If you are a rookie make sure you don't overdo them! Sometimes it gets a while for them to get going, but once they start cooking it can be a very short time between rare, medium rare, and then (way too) well done. Make sure you ask your guests how they like them and put the medium ones on first, with the rarest going on last. Also know your barbeque, some spots can be hotter than others. A lot of it is just trial and error.