Hands down get a Thermapen. Now is the time to buy one ($59). You won't find them any cheaper than this unless you buy one used. It is my most used accessory and I must say has been solely responsible for elevating my culinary skills to another level.
http://www.thermoworks.com/Classic-Thermapen
this for starting off ...before multiple cooks will allow for eyeballing sear vs thickness to be able to eyeball gauge the medium rare target.. and the current 59 sale on the classic thermapen is a steal of a price..