Have a problem...
Sometimes when i shift the gear will not engage and take a little while, sometimes the gear will change and the chain seems to rub on the derailer. any quick fixed?
There are a couple of adjusting screws down on the derailer. Adjust the stop screw, but if you adjust too much your chain will jump off the sprocket; its one of those play with it until it works things. It also depends on which rear sprocket your chain is on at the time of the shift on the front sprocket.
You just need to do a rear derailleur adjustment. It's a fairly simple task, you should be able to find instructions on the internet or in any basic bike repair book. If you google it you'll probably get more info than you want.
If it's safe to assume this is the front...
...derailleur then it's most likely a cable tension issue. The two limit screws do just that-they adjust the outer limits in which the derailleur can travel. As was mentioned above, you want the H limit adjusted so that you can shift up to the large ring without shifting off of it into your crank, while the L limit is adjusted so that the chain can drop into the small ring without dropping into frame.
If the chain is hanging up moving from one ring to a larger ring you need to increase the cable tension by turning the barrel adjuster at the shifter.
If the chain won't drop from one ring to a smaller ring you need to decrease the cable tension at the barrel adjuster.
Shift cables stretch, particularly new ones, and need to be adjusted. Thankfully most of the stretching occurs in first couple of rides.
This site can be a lot of help: