Word もっと is pronuncing as motto means more. But when writing It is mo(も) tsu(っ) to(と).
Why is that ? I'm new to Japanese language.
Please tell me there are other commonly used words like this ? Please give me some examples.
This is a double consonant sound. It's denoted by the smaller size つ. So instead of the word being pronounced as Mo-tsu-to, it is pronounced Mot-to because of the っ. That is why you have two t's instead of just Mo-to. This is similar to かった in which the actual pronunciation is kat-ta instead of ka-tsu-ta. Another consonant sound is added before っ that is similar to the one following it. That is why in both もっと and かった the tsu is replaced by a "t" because the kana following っ are と and た, which both start with a "t" sound. This is why it is called a double consonant sound.
That character is not exactly the tsu-character!
The second character of もっと is called sokuon, it's not the normal hiragana 'つ'. The difference is that the sokuon is smaller (other than that, they are identical, so your question is totally understandable). For this reason it is also called small-tsu (or chiisaitsu in Japanese).
The sokuon is used to carry the consonant sound of the second character to the end of the first. In rōmaji, that is represented by doubling the consonant, that's why もっと is romanized as motto. This change in pronounciation (from moto to motto) is not at all trivial, and it takes some time to get used to it.
There are other characters in Japanese that are small versions of others. There are, for example, the small 'ya' (ゃ), 'yu' (ゅ), 'yo' (ょ), used to create sounds like 'kya' (きゃ), 'shu' (しゅ), 'cho' (ちょ), and so on. This also happens in katakana, not only hiragana. Since you are a beginner, I totally recommend reading this: Tae Kim's Guide to Japanese - Hiragana. It covers all those things about hiragana, and it's very well written (that's exactly what I used to learn!)