I want to make a programing language

What would be the best way to do this?

10 PRINT "SPL SHITTY PROGRAMMING LANGUAGE INTERPRETER"
11 PRINT "VERSION -32767 07-07-2007"
12 PRINT "(C)2007 >>38, RELEASED UNDER GPL"
20 PRINT "BYTES FREE=";FRE(0)
100 INPUT ">";C$
101 IF C$="EXIT" THEN SYSTEM
102 IF C$="RUN" THEN PRINT "PROGRAM NOT DEFINED"
103 IF C$="FREEMEM" THEN PRINT FRE(0)
104 IF C$="SAVE" THEN PRINT "PROGRAM 100% SAVED RELIABLY"
110 PRINT "SYNTAX ERROR"
111 GOTO 100