Replacing a light fixture

This is much simpler than it sounds, but if you're a total novice you'll need to read it all.

First, make sure power to fixture is off. Best method is to locate circuit breaker or fuse that controls that fixture's power. At minimum shut off the wall switch to it if there is one.

Next, remove the old fixture. Usually there are two screws or decorative nuts holding the base cover (canopy) on. On ceiling pendant fixtures there may be a retaining ring holding up the canopy, unscrew this ring counterclockwise and the canopy will drop over the support chain, exposing the junction box where the wires are connected.

It is critical at this point that the power is off.

Modern house wiring and fixtures are connected with three wires.
Green or bare copper = ground (earth)
White or grey = neutral
Black, Red, Blue or any other solid color other than green, white or grey = power or hot wire (this is the wire that is controlled by a switch.
For simplicity I will call the three: Green, White and Black.

The next step requires you to carefully pull the wires out of the junction box and isolate the black, white and green wire connections from each other. Make sure the connectors (wire nuts) for each connection are accessible.

Warning: With old or over heated wiring, take extra care not to crack or disturb the insulation(wire covering) on any of the wires. If insulation is crumbling away exposing bare wire, stop and call an electrician! Do not turn the power back on until he has repaired those damaged wires, they could start a fire.

Isolate the connection between the black house wire and black fixture wire. Remove this wire nut. At this point I usually use an electrical tester to double check that the power is off by touching one side to the newly exposed black connection and any grounded metal part nearby.
(Note: some multiple switched (three way) circuits may use a white wire as a power wire. If the black fixture wire is connected to a white house wire, mark that white house wire with black electrical tape for future identification.)
Once you are positive there is no power at the fixture junction box remove the connectors from the white and green wires.

Detach the old fixture and it's mounting bracket.

Re-attach the new fixture using bracket and new connectors that are usually provided. Some brackets have a green ground screw. Wrap the bare ground wire clockwise around that screw and tighten it down leaving enough remaining ground wire to attach to the fixture ground.

If the wires to the new fixtures do not have a 1/2 inch of exposed bare wire at the ends (stripped) you must do so. I use a wire stripping tool but you can do it with a sharp knife being careful not to nick the metal wire.
Once the wire tips are stripped, hold the tips of each color pair together side by side, slip the wire nut over them and twist clockwise until snug. (White to white, black to black etc.)
Take care that the new wire nuts are secure at each connection. Do not over tighten them but insure that they are correctly attached by gently tugging on each wire. When completed the black and white connections should have no exposed bare wire showing.
The Green (bare copper) wire is there for safety and never carries current, hence exposed wire is not an issue on ground wires.

Assemble and attach the new fixture according to instructions in the box.

Pendant ceiling fixtures usually require additional assembly steps including adjusting chain height, looping wires through the chain and slipping retaining nut and canopy over chain prior to connecting. Always follow directions that come with the fixtures.

When in doubt, call a professional. Electricity is dangerous.