First off, stop listening to your fish store. If your stuff keeps dying, and they are telling you what to buy, and that things are fine, they are obviously steering you wrong somewhere. They are making money off of you.
You need to buy your own test kits. You need test kits for ammonia, nitrite, nitrate, pH and alkalinity. You need a thermometer to check your temperatures. And you need a hydrometer or refractometer to test your salinity.
Why would you trust the people that you are giving money to to give you advice? They know if you buy something and it dies, you will just come back to them and buy more stuff to replace it. I'm not saying they are purposely duping you, just that this is YOUR tank, and YOU need to be in control of it and understand what's happening in it.
Next, there's no way you should have had sea horses in that tank. They require very special habitats tailored specifically for them. Unless you planned and set up your tank exactly for the needs of seahorses, then of course they're going to die. They're not like other fish that you can just toss in the tank and hope they survive. They require different temperatures, different lighting, and different foods than other fish. Most will only eat live foods unless specifically trained to frozen. If they sold you seahorses and told you those would be okay for your tank, a beginner tank, then they flat out lied and stole your money. Seahorses are not fine for a brand new beginner tank.
So start off by buying those test kits and finding out for yourself why your stuff is dying. We can't tell you why without knowing what your water parameters are. This is your money you're putting into this hobby. Know what you are spending it on! You shouldn't be okay with being blindfolded and led around by someone else that may not have your best interest in mind.
Next, how are you acclimating these animals to your tank? Are you just putting them in when you buy them, or are you using a proper acclimation procedure?
Lastly, are you using tap water? Or reverse osmosis deionized (RODI) water?
Edit: Also you need to buy a copper test and test your tank. I know you have a shrimp and a starfish, but I'm thinking they may have sold you contaminated rock.
AND do you have enough powerheads for water movement in the tank?
AND what kind of lights do you have?
AND do you have a protein skimmer?
Don't worry, we'll get to the bottom of this. ;)