It is quite usual that when a user first goes out with a metal detector he or she will return with masses of SCRAP! The reason for this is quite simple... the machine makes the slightest beep and the user digs without having the experience of knowing what the beep is telling him/her. Consequently the first time user ends up digging everything, thereby finding mostly scrap.
Speaking from experience we find that after a couple of trips out with a metal detector a user will begin to associate various sounds and meter readings with items of scrap and consequently will begin to dig up less scrap items.
What else can be found depends primarily on what type of ground is being searched. Parkland will produce mostly modern coinage and artefacts, beaches the same with a chance of a piece of jewellery. Farmland is likely to be either the most barren area or the most interesting, it depends what use the field has seen. A field that has seen little use save for the odd farmhand walking across it will produce little if anything whereas a field may have had a Roman settlement on it and produce thousands of coins and plenty of artefacts.
Of course, a wonderful find can pop up anywhere so.... best of luck!!