Assassin is correct. You need to break free of your English language mental models in order to learn a new language. It's not easy, but the more lanuages you pick up and the more you speak, it gets easier breaking free of these mindsets.

Anyway, just to help you out in the 2 sentences above.

uma　 ni 　notte iru 　onnanoko
馬　　に　乗っている　女の子。
horse (particle) riding girl.

'Uma' means horse, 'ni' is the particle, 'notte iru' comes from the verb 'norimasu', which means ride. 'Notte iru' means riding. 'Onna' is woman, 'onna no ko' means girl.

teeburu 　no 　shita ni iru 　otokonoko
てーブル　の　下にいる　　　男の子
Table (particle) under (particle) boy.

'Teeburu', written in katakana, means table obviously. 'No' can be translated to 'the', 'shita' means under, 'ni iru' comes from 'ni imasu' and refers to the place the person is at. 'Otoko' means man, 'otokonoko' means boy.

Japanese isn't easy, but you get satisfaction when you can understand anime without the need for subtitles, or go shopping in the Japanese section and understanding exactly what's written on the packaging.

My advice is start with memorizing the hiragana and katakana first. Kanji is inevitable but there's too much to memorize so don't get confused by it. You'll be surprised at how much you can read just with the Katakana alone. And of course, don't give up!