Ok why is staying in the EU a good thing:-
Trade is becoming more global, that's not a suggestion or an idea it's a fact. Countries that supply goods and services get better trade with their immediate neighbours from being in trade blocks, unless you a huge country with a vast population that can supply and manufacture all your own needs. No country falls in to that category but some e.g. Russia and USA have more resources than most.
So here's a bit of a background to why in the first place - this will drone on so apologies. To be able to properly compete with those countries you need to have a similar leverage in the market place, this is easily achieved by drawing up agreements with your neighbours to help you negotiate with those bigger countries.
Similarly your neighbours may be brilliant and making cars, but not so good at the financial side of thinks, or farming for example. So what you do is sign an agreement that allows your goods and services to be traded with theirs. You also notice that you are both pretty good at producing steel and aircraft. So what you do is have an agreement that makes sure the other country can't have an advantage over you in steel by paying their own producers more money to make the steel more profitable for them.
Hence you have harmonisation over the laws that govern farming, steel production, the rights of workers allowing your specialists in banking to go abroad to work in their country and their skilled car engineers to come to yours.
This gives more strength when negotiating deals with other countries. Now admittedly the stronger you are the more you can negotiate deals to your advantage and that is what the EU does, hence the wealth of trade agreements opening up our export markets in our strong points to other countries. You carry on improving the rules for trade between you, whilst still allowing the governments to make their own laws and to run their own economies. This means that within the trade bloc you are a member of, you have free access to those markets with which you can do the easiest trade. You use your collective power to negotiate trade deals with other countries, and stop the bigger countries from bullying you (have you looked at American food standards - ye goods they're horrifically bad).
You vote for representative to go and make laws to protect your position. You carry on refining the process so that within your bloc you have equality. That's where we are now.
So staying in, we are part of one of the biggest trading blocs in the world, we have the power to negotiate with any other country or group of countries.
The bloc we are in will continue to subsidise the areas of the country that we forget about by investing in infrastructure and supporting projects. We pay less to be a member of this block than we could do having to do all this ourselves, we don't have to negotiate with countries that are bigger and more powerful than us (and if anyone sees how trump is behaving take heed). We get a say in the future direction it takes, and can opt out of bits we don' like (Schengen).
It also helps to ensure peace with your neighbours.
Those are the advantages in a simple format. There are no advantages to leaving - none at all.