Customizing and upgrading your weapons can be done in any number of ways. To increase the amount of Raritanium you acquire immediately, however, try upgrading your weapons initially in terms of their Raritanium output when used. Since many enemies give up Raritanium when killed, you can make them drop more by using weapons upgraded to increase Raritanium output. Upgrade the output before upgrading anything else, and you'll have plenty with which to upgrade before long!