With a 15Khz arcade monitor you definitely want to use Direct Draw with Hwstretch.
If you are using commandline mame, both options are in the mame.ini file which should be in your mame folder, simply replace the '0' next to each entry you want to turn on with a '1' then re-save the ini.
As ulaolao said though, if you're using mame32(UI) then go to options from the toolbar at the top, select default game options, then change them in the display settings tab.