I bought a Rocketfish 55mm uv filter and tried to install in on my 18-55mm lens. I thought it would be easy but I think it's a wrong fit. I need help installing it.

Sometime the filter gets screwed on too tight. If you can't remove it there is a tool available that grips the outside of the filter tight and allows you to unscrew it. Google filter tool. Remember...Lefty loosey Righty tighty.

The step-up ring will on any lens with a 49mm filter thread, regardless of the manufacturer. The easiest way to find out whether it works with any given lens is to see whether the ring fits onto the front of the lens (you could also read the filter size written on the lens itself).
If it doesn't fit, purchase the appropriate step-up ring. That will be much cheaper than buying a whole new set of filters.
If the new lens(es) have a 55mm filter thread then you don't need the step-up ring at all. The filters will mount directly onto the lens.

Just a thought - are your Image Stabilizer and Auto Focus switches On or Off?
Might be worth experimenting with them.
Otherwise, sorry, can't help.
May be necessary to contact your vendor.

Not directly. The 18-55mm takes 52mm filters. However, you could purchase a 52-58 step-up ring to allow you to adapt the filter to the lens for significantly less money than a 52mm UV filter. One such adapter is http://www.adorama.com/IRXRA5852.html

They told you wrong. The 18-55mm lens takes 52mm filters, as is marked on the lens near the mount.

You can either trade your 55mm filters for 52mm filters, or else get a 52-55mm step-up ring, which will adapt a 55mm filter for a 52mm lens.

Filters go onto the front of the lens. Remove the lens hood if one is attached. You'll see a threaded rim around the lens. The correctly sized filter (or step-up ring) screws into that.

52mm

the les coattings might have damaged due to uv filter broken,better get it the lens serviced.

The Canon lenses are FD mount which means that you will need to get an adaptor to convert the FD mount to fit the F mount on the Nikon. An example of the mount can be seen here:

http://www.rugift.com/photocameras/canon-fd-nikon-adapter.htm

As for the speedlights the hotshoe mount will accept them on your Nikon, put them on with fresh batteries and try them out.

THERE is a problem in lens not with the UV filter
remove the lens clean the contact of lens & camera if not working repair or replace it

If it is the EF-S 18-55 mm lens, the filter diameter is 58 mm.