Watashi-wa, ichinensei-desu. わたしはいちねんせいです。

=I am in the first grade.

watashi=I, me

(noun)+wa="Wa" makes (noun) subject of the sentence.

ichi=1

nen=year

(number)+nensei=(number's) grade (at school)

desu=be-verb in present tense

Sentences are structured basically in this way in Japanese:

Subject+Object/Complement+Verb.

In the sentence above,

"watashi-wa" makes subject, "ichinensei" makes complement, and "desu" makes verb.

Numbers in Japanese:

1 ichi

2 ni

3 san

4 yon, shi

5 go

6 loku

7 shichi, nana

8 hachi

9 ku, kyuh

10 juh, toh

11 juhichi

12 juhni

20 nijuh

21 nijuhichi

22 nijuhni

100 hyaku

101 hyakuichi

102 hyakuni

1000 sen

10000 ichi man