For another example, wǒ xiǎng wèn nǐ, means I want to ask you - Simple enough. But, if you were to say wǒ xiǎng wěn nǐ, it would mean I want to kiss you! The only difference is the tone over the e in wen! That could be a bit scary LOL.
Altogether there are over 50,000 characters in Mandarin Chinese, however, a modern dictionary will only list about 20,000, and you only need to know approximately 2000 to be able to read a newspaper.
千里之行，始于足下 Qiān lǐ zhī xíng, shǐ yú zú xià
A journey of a thousand miles starts with one step. Laozi, Daoist philosopher (6th Century BC)
One to bear in mind as you take your first steps in the Chinese language!