How can you contact spirits?
Spirits are those who have passed on to the Spirit World. We ourselves cannot contact them and they cannot contact us unless God gives them a special message for us. They will deliver their message but will not stay to chat.
One may ask God to let you know if they are alright and the answer may come in a dream. But there is no one on one conversation held.
Without going into the complex realm of dimensions those that have died merely inhabit a different dimension/reality. There is a possibility of communication but should only be breached with caution, knowledge and respect by those learned in this ability.
There are some who are naturally gifted people who are capable of communing with those who have passed, these are called mediums. You can pay for some, but usually if they are pretty sincere they will help free of charge.
You can also contact spirits through EVP's or Electronic Voice Phenomena which is where a spirits voice is captured on a voice recorder and there is no explanation as to where the audio came from. You can check different ghost hunting groups for more information on this.
Yet another possibility to consider is going into trance. When you go into a trance like state, or level of deep meditation, you open your mind and body to receive messages whether from the Divine (if you believe in such) or spirits. This of course is one of many methods, but experimenting with different ways can help you find your happy medium at some point.