Conjugating verbs to the negative present tense in the plain form varies depending on the verb's ending. There are three types of endings we need to pay attention to:

Group 2, or 一iti段dan verbs, which all end in -る
Group 1, or 五go段dan verbs that end in -う
Group 1, or 五go段dan verbs that do not end in -う
The irregular verbs です、ある、する、来る

For 一iti段dan verbs, replace the -る with -ない. For example, 食べる becomes 食べない.

For 五go段dan verbs that end in -う, change the -う into -わない. For example, 買う becomes 買わない.

For 五go段dan verbs not ending in -う, change the う-sound into an -あ-sound, and add ない. For example, 行iく becomes 行iかない.

And finally, the negative plain form for the irregular verbs is as follows:

Plain Form Positive	Plain Form Negative
だ	じゃない
ある	ない
する	しない
来る	来koない

And here are example sentences for each of them:

Breakdown of kanozyo no koe ha utukusii kedo, utawanai.