First up uncivilized nations get two new types of reforms: Military and Economic, replacing the old reforms. To fully embrace westernization you need to pass a certain amount of these (either the entire military track, or the entire Economic, but mixing it up also works). Different reforms have very different effects. Some, like Army Schools, gives a boost to starting experience of all troops where as International Debt lets you take loans from other nations and Improved Transportation System give you free railroads constructed in your capital state. Enacting unciv reforms cost research points (modified a bit by upper-house composition), and it is no longer possible for uncivs to research technology through the regular trees. As you progress towards westernization another nice flavour thing is that your country's label changes. You can go from primitive, uncivilized, partially westernized to fully civilized.
Reaching 100% westernization progress unlocks the action to westernize. When you do you will become a civilized nation and given technology. What technology you get depends on how you have decided to enact reforms (mainly military or mainly economic?) as well as if you are in a sphere of influence. If you are in a sphere you will be given technology based on your sphere master. If you are not in a sphere it is instead based on the lowest common technologies among the great powers. Being in a sphere also affects your westernization progress as your masters tech school will make different reform tracks easier. So for example if you are in Prussia's sphere military reforms will cost less to enact but you will get a bit costlier economic reforms. To give you as an uncivilized nation some little choice in the matter of sphering, good relations now give a small boost to influence so your friendly neighborhood great power are more likely to influence and win any competition of influence.
Uncivilized nations also get a new type of rebel faction that long for the good old days of pointy sticks and who are not impressed at all by your new railroads. If these rebels rise to power they will reverse reforms and whenever you enact a new reform you will anger the conservatives/reactionary risking that more people join this faction.
To have a chance to present all the new tanks you are getting double the tanky goodness this week. We have the French 22 ton Saint-Chamond tank prototyped in 1916 and the later 6 ton MS-1 tank produced in 1928-1931.