a few tips ill give you which people seem to write on here with mistakes about are;
make sure your tank is cycled properly and do it with a fishless cycle so many people buy expensive fish that die or damsels that they can't get rid of.
Do lots and lots of reasearch on compatibilty and tank sizes before you buy any fish so you dont end up with a trigger eating your favourite clownfish or a tang in a 20 gallon tank and biggest tip of all lfs
lie a lot to sell you anything! this doesnt mean to say all your research will be right as every fish as every person is different. for example I have heard of people keeping trigger fish with lion fish but also heard of them ripping them apart sometimes its luck of the draw. best place I use for my information is on Liveraquria they usually go a little on the causious side so if you go by them shouldn't go wrong until you have more experience to know differently.
one that I learnt from mistakes is to not use tap water even with the chemicals that make it safe and to ensure that you top up your tank with fresh water to conteract evaporation buy good test kits too as first thing people on here will ask when you have a problem is what are your parameters
last one as i have realised I am going on a lot is that less is more in saltwater as these fish need room to swim grow and just need good water. this means putting less fish in and increasing water volume with sumps etc will help you keep your tank in shape. this should definitley be something you think about when adding too as if you add all your fish at once yo will over load the tank be sure to take time on it.
hope all this helps but if you hve any specific questions just pop ut on here sure me or someone else will be happy to help if the can