I have found many articles on why America entered ww1 and the thought of fighting America but I haven't been able to find the reaction when America announced its entry. What did the citizens of allied countries think and say? What about the citizens of the enemy? What was the reaction to the soldiers on both sides?
I guess we have to start with the domestic reaction within the US, which took years of posting pro-allied news stories in the paper in order to way public opinion towards supporting the war effort.
Germany's actions certainly didn't help in dissuading Americans from supporting entry into the war; with the sinking of the Lusitania, and later, the public release of the Zimmerman Telegram was the final straw that drew outrage from most Americans. That along with President Wilson's overtures of self-determination of peoples and the preservation of democracy brought the American public to support the call to arms.
a NY Times article announcing entry into the war can be found here:
http://www.nytimes.com/learning/general/onthisday/big/0402.html#article
As for the German reaction, first you have to start with German hopes that the US could be a mediator for peace. With the Central Powers making peace overtures to the Allies in December of 1916. Germany sought President Wilson's counsel as a possible neutral mediator (an "honest broker") to the peace, but still held distrust as American opinion was souring quickly over Germany's past submarine attacks on American merchant shipping.
Wilson did agree to a pre-mediation of sorts and asked both sides to state their demands for peace. The Allies demanded evacuation of occupied territories, war reparations for France, Russia, and Romania, and a recognition of nationalities living in Central Powers territory, including the Italians, Croatians, and Slovaks in Austria-Hungary and the Poles living in Germany. These demands were outright rejected by the Central Powers as this would force Austria-Hungary's collapse if their nationalities were given more recognition, as Serbian nationalism in the empire was seen as the spark that blew the powder keg in the first place.
a good reading into the demands and mediation can be found here, Page 721-766:
https://www.jstor.org/stable/2637713?seq=1#page_scan_tab_contents
The Allies as well were weary of President Wilson's calls for self-determination of peoples as a condition for peace, which would place the future of Britain, France, and Italy's colonial empires into question.
When Wilson did enter the war on the side of the allies after the Zimmerman Telegram, the Germans mostly saw it as inevitable after the December, 1916 mediation, where Germany decided to resume the policy of Unrestricted Submarine Warfare in January 1917, before the US even voted to declare war, reasoning that the US is going to declare war anyway in the near future. So they sought to mitigate American economic power to the allies as much as possible even before then. A great book detailing the events of the Zimmerman Telegram and German preparation for American entry can be read here:
The Germans thought the strategy of using submarine warfare to mitigate American economic power to the Allies would work, as the Americans were seen as powerful economically, but rather weak militarily, with a standing army of around 100,000 at the start of the war. The Central Powers concluded that if the war can be wrapped up before the US can bring its full economic and rapidly building military might to bear, they could still cut out a favorable peace deal if they could still knock France and Russia out of the war, which they succeeded in removing Russia.
The Allies reaction to US entry however was actually pretty mixed. The Allied soldiers and commanders themselves were extremely grateful for the manpower and resources of the US finally coming to aid the Allied war effort, but the Allied leaders saw Wilson's calls for democracy and self-determination as a threat to their territorial claims which included conquering pieces of Austro-Hungarian, Ottoman, and German land. Wilson's 14 points were accepted by the Allies on paper, but that didn't stop the land carving by the Allies across Europe, Africa, and the Middle East.
I can't find an online version of this, but a good read related to Wilson's influence at the peace talks can be found at: Hakim, Joy (2005). War, Peace, and All That Jazz. New York: Oxford University Press. (p. 16-20)