UMDGen is primarily used to
rebuild previously dumped and extracted UMD iSO images for use on a PSP Memory Stick. With that being said, if you are just looking to back-up a UMD Game to your Memory Stick and playback without the original UMD then UMDGen isn't required.
HERE is a thread that details how to use the various PSP dump utilities (all found in our
PSP Utilities STICKY thread) to dump your UMD discs
from your PSP drive to your PSP Memory Stick. For now, you'll need a PSP with firmware version 1.00 (JAP) or v1.50 (USA) to run the dumped game images (PSP iSOs).
THIS thread will show you with pictures how to identify the PSP Firmware Version from the outside of the box... so you don't end up buying one that won't work.
As for how to run the iSOs from your Memory Stick to play the games, you'll need to
read for an hour or two... I suggest checking out some of the related Tutorials in our
PSP Tutorials STICKY thread, for example,
How_To_Run_Homebrew_Applications_On_PSP_v1.50.zip and it probably wouldn't hurt to read the
PSP_UMD_Dumping_Guide.zip there either... or any others that interest you.
Finally, games vary in space required on a Memory Stick. Most are from 300MB to 1GB in size, but some are more or less, and the larger ones can be ripped down to fit on a smaller Memory Stick. You definitely aren't ready to get into doing PSP UMD Rips (which UMDGen can be used to create) as it's clear from your post you haven't even ran a non-Ripped PSP game iSO on your PSP yet... so crawl before you try walking, and don't be LAZY and not
SEARCH for the information you're seeking before posting a slew of redundant questions. Good Luck!