www8.garmin.com/manuals/GNS430_PilotsGuide.pdf
Go to the user waypoint page, activate the cursor by pushing the small, inner knob, and enter the name of your waypoint.
Press ENT.
Your present position will appear in the lat/long box at the bottom of the screen, if you're getting a GPS signal that gives you that position. If you want to save the present position as your waypoint, use the large knob to move the cursor over to CREATE, and press ENT.
If instead you want to enter a different LAT/LONG, then use the large knob to highlight the POSITIONfield. Use the large and small knobs to enter the position you want. Then use the large knob to highlight CREATE, and press ENT.
When you're done, press the small knob to remove the cursor. The waypoint is now in memory, and you can establish other waypoints from it, fly direct to it, or whatever you wish.
If you'd rather create a waypoint off an existing waypoint (such as a hold off a VOR radial, for example), rather than highlighting the POSITION box, you can highlight the REF WPT box and enter your information there.
The guide linked above will give you step-by-step instruction.