Internet is the best resource for political information, especially if it's from a third party source unrelated to the actual event:
http://www.theweek.co.uk/eu-referendumBasically the argument is this:
1. If Britain leaves the EU it will weaken Britain's economical standing, they won't be able to participate in cheap trade with other EU countries, Britain investments will go down causing the country to be worth less and Britain won't be part of these influential deals with the US. In short the economy of Britain will suffer, but the degree of suffering is not well-known, some people say that Britain can hold it's own as a solitary economic power not part of the EU, but whether this is true is up to speculation.
2. If Britain stays with the EU Britain will be subject to the laws of the EU, meaning that if the EU passes a law and Britain passes a different law, Britain is forced to change their laws. This means Britain is at the mercy of the EU like every other EU country, and thus has less legal freedom. The most controversial aspect surrounding Britain at the time is immigration laws. In the EU, all member states must have open borders with all other member states, this means a multitude of European immigrants can come into Britain and legally there's nothing Britain can do about it. This can potentially cause distabilization seen in such eras as the Syrian Refugee crisis.
So don't listen to political parties, listen to third party sources.