You can say like that： 大家好。da jia hao. 我是法国人。 wo shi fa guo ren. 明年我要去中国的上海。 ming nian wo yao qu zhong guo de shang hai. 可是我的中文说得不是很好。ke shi wo de zhong wen shuo de bu shi hen hao. 为了和中国人交流，我要提高我的汉语水平。 wei le he zhong guo ren jiao liu, wo yao ti gao wo de han yu shui ping.

我爱你（or 我爱你们），新年快乐！ wo ai ni (wo ai ni men) , xin nian kuai le!

haha, I made a mistake. Actually, it is also difficult for us to pronounce. "er" sounds （儿化音） is one of the bigest balk to Hokkein when we take Spoken Mandarin Level Test （普通话等级考试）. And also most people from south China. By the way , the test is a indispensable recognition for people want to be teachers ，performer, comperes, announcer etc. Never mind if you can't speak retroflex final

It depend on whom you trade with. If your major contracts are from mainland or taiwan, you'd better choose Mandarin. If most of your trade partners come from HongKong, you should study Cantonese. Basiclly, I suggest leaning Mandarin.

Meanings won't changed wheather you add "er". It comes from idomatic code of language in north China, especially dialect spoken in and around Beijing. Actually, it is also difficult from us to pronounce. P.S. I live in Fujian(Hokkein).

事实上就专业上说，两者都是可以的。当然标准的读音是shuí。shéi是音变。但是任何语言都有音变，比如英语中sk、sp的读音接近sg、sb。 有些音变是语流音变，就是某些字或词在单独念时的读音和在语句中的读音不同。最简单的例子，老虎的老，本来应该读三声，但放在同为三声的虎字之前发音就变为接近二声。还有“一下子”的“一”发音就要比通常的发音短促。 还有一些是任意音变，谁就是其一。shéi 和 shuí 都是正确的。还有是为什么，可以念 wei shen me，也可以念 vei shen me。

Mandarin is special, because Chinese character isn't phonography.Ttranscribing will be necessary. All of us do that when we were schoolchilden. Taking lessons will be useful. Teachers can teach you grammar what a native sperker without professional training can't give you.

你好(ni hao) or 您好(nin hao). the former is said to the same generation or junior. the latter is to elders and betters