Since you are new at fly fishing it is probably your cast but you have to look at everything. Here are some ideas.
1. Make sure your leader is matched to the fly you are casting. Take a look at this chart and make sure the fly is not too large for the leader tippet.
Here is a very handy chart that may help you out.
2. If the tippet and fly match you might try a shorter leader until you get things shorted out.
3. Most new fly fishers cast too quickly. the fore or back cast don't unroll completely and you loose your line speed. When I have a problem with a leader here is what I do:
If you are on a stream find a calm pool with no currant. Get your line out in front of you laying on the water. I do this with a roll cast. When you have about 30' if line on the water do a Pick-Up and Lay-Down. You start the Puck-up with the rod tip low to the water. The fly line should be locked against the rod grip with the fore finger on the rod hand. You retrieve some line until the tip of the fly line is moving. Now do a Pick-Up and into your back cast. When you feel the rod load you start your fore cast and let the line settle on the water. So you do a pickup, a back cast and then on the fore cast you delver the line as if you are fishing. This abbreviated Pick-Up and Lay-Down will show you if your leader is laying out straight or collapsing.
The Pick-Up and Lay-Down will tell you if the problem is the leader or your casting. In many cases a beginner can do a Pick-Up and Lay-Down with no problem. It is when they start to false cast that the rush the cast, loose their line speed and everything goes to the devil.