I am wiring a simple one bulb fixture in a closet. The old fixture had a 3-prong outlet plug as well as a bulb and the new fixture only has the bulb. There are two black and two neutral wires (for a total of 4 wires coming from ceiling) but only two screws on the new fixture. Can I combine the two black to one screw and two neutral to the other or will that supply too much power to the 60 W bulb?

Don't put 2 wires under 1 screw. Add a small pigtail wire, 1 black and 1 white to the existing wires using the wirenut. This will give you 1 wire for each terminal. No problem with a 60 watt bulb but if it is a bare bulb, consider a CFL in a closet as it produces very little haet and reduces the possibility of fire due to clothing on the bulb. New codes do not allow bare bulbs in closets.

First, is there a wall switch for this light or does the fixture have a pull switch on it.