First you should know that a nikah is valid if all conditions are met (For details refer to Is a marriage without a written contract and a late dowry valid? and the provided evidences and links). And to declare a nikah as invalid because you have been forced to it you must first go to a shari'a court and ask for faskh (annulation) and depending on the reasons you give them maybe you need to prove that you were forced. Also note that the longer you stay inactive the less your chances for an anullation will be. Scholars say that forced acts are invalid, but this doesn't mean that you can apply the ruling without telling the authorities. As you have got married in front of witnesses and the agreement of your guardian you should also get "free" from this marriage in a legal manner!
Note that separating two married people is considered a sinful act in Islam. Therefore the initiative for this should come from you and if you fail to get an annulation of the marriage you may also have the option of khul'a which is buying yourself free from this marriage in this case you should know that you must pay your husband an amount on which both of you agree.
Of course you could get a divorce if your husband utters the words of talaq, but he would have a chance of reconciliation within the 'iddah. If divorce (talaq) takes place you would be free without any "costs", but talaq and rocniclliation are in the hand of the husband (See One talaak to wife and want reconciliation, what do I need to do?)
