You have a number of options to consider.
Before even choosing your source you need to think about how many visitors you plan to have. If you dont plan to have many (say less than 10000 peers) then you will be able to use a php/mysql only tracker which can run on most shared hosting. ie. hosting you dont have root access to. DO NOT use free hosting. They will have either stopped their service being used for torrents or will boot you as soon as they notice the high bandwidth usage.
Otherwise you should go with a tracker that uses
XBT as a backend. This is a tracker which is written in C++ making it much faster than a PHP based tracker. Millions of peers are seemingly possible using XBT on a single server.
Next up is choosing your source (there is lots so i will just point out the ones I know of well)
The most popular source is
TBDev but this is well known for its security exploits and doesnt seem to be updated much. Saying that, it has a very active forum with plenty of modifications to choose from. Standard tbdev doesnt have XBT support but there is a modification for it in the forums which Im sure many trackers are using.
Next up is
XBTIT which is written using PHP & MYSQL. This has XBT support 'straight out the box' which may interest you if you have a server and are looking to make a popular website with minimum hassle. Once again the forums here are busy and helpful. They also offer a paid support service & hosting.
Yet another is
Torrent Trader which is another PHP/MYQL tracker. I dont know much about this and cant get to the website at the moment so cant investigate. Try look for yourself! If memory serves they have a basic forum but to get any decent modifications etc you need to pay a subscription, but this may have changed.
There is lots more, plus lots of users modified versions of the 3 above. Your best bet is to have a look around their forums and google to see what interests you.
Then you need to carfeully choose the location of your hosting, as depending what content you are planning to index, it may not be legal in your country of choice (or it may be legal, but money will sway the authorities judgement, you know the deal). That is up to you to determine.
After all that, upload your files and away you go!
Oh yeah, it helps to know HTML, PHP & MYSQL as a bare minimum. Linux is a definite bonus too (unless you plan to use a Windows server)