You dont need to run the rrdtool create command, MRTG will do this for you automatically, and it will do it in the correct format.
You will need some sort of display frontend -- I suggest Routers2.cgi (of course I would
) but you could use 14all or mrtg-rrd instead.To collect the data depends on how it is available. If it is available via SNMP you can simply specify the OIDs in the Target definition, but if it needs some other method you'll need to write a data collection plugin. This can be in Pelr or in any other language that you know; if you dont know a programming language then youll need to get a book on it to learn! The help on all this is available in the MRTG website, along with the specifications for a data collection plugin.
A data collection plugin should
1) exit with status 0
2) be runnable by the MRTG process user with the default environment
3) Output on stdout "A\nB\bC\nD\n" where A and B are the numerical values, C is an optional uptime string, and D is an optional service description. C and D are usually left blank when running in RRDTool mode as they are not used then anyway.
You can see the MRTG documentation for the Target directive with more information about how to specify a target.