The church externally presents the typical aspect of medieval architecture of Florence: solid and simple, made with local yellow stone (pietra forte), a simple kind of stone, not refined. A lot would seem a confidential church, focused on the inside, but the three enormous decorated windows of the apse let the light shine, as beautiful as innovative considering the time of the ages and and height of the openings too.
The inside is a real masterpiece of gothic architecture: I mean the italian gothic, if you're looking for things like Notre Dame de Paris you could be disappointed, because that is french gothic which is quite different. Italian gothic was born before french one and has established canons and rules of the style; in France then the style has been reinterpretated in a very characteristic way. The gothic style of Santa Maria Novella is sober, strict and essential, ornaments are not elaborated sculptures and hazarous vaults, but it is adorned with the most simple and true qualities: geometry, the contrasting colors of the materials, an incredible constructive skill.
The plan is cross shaped, with three aisles and pointed arches. Arches are decorated with alternated green and white stones. The spans as they are approached all' apse reduces own width, increasing the perspective effect and giving the illusion that the aisles are longer than they really are. This testifies the huge skills of architects of 1200 AD: a careful study of the perspective, in an age in which it was still unknown. Over the altar has been recently replaced the famous wooden Christ by Brunelleschi, eventually restored. The ambient inside is wonderful in its simple perfection given from geometry, and is more lightful thanks to the three great windows of the apse.
The facade by Leon Battista Alberti represents the synthesis of all the ideals of beauty in renaissance: a modular, geometric facade, organized with geometric rules and proportions. The two lateral curls in the upper part of the facade are an addition stuck there in baroque age, try to imagine the facade without the curls and find the perfect proportions.
Santa Maria Novella is not only a church, is a complete monastic complex with a cemetary, a botanical garden, the refectory and various chiostri. Also these places are wonderful and decorated with spectacular frescoes and sure deserve a visit.
There is also an ancient herbalist shop runned by friars since 1500 AD, with lots of typical herbal products and wonderful ancient furniture and jars.
Here's a video guide that I found on youtube of the church of Santa Maria Novella, very useful for your tour!