Formal:
* Мне нужна помощь. (Mne nuzhna pomoshch.) - This is the most direct and formal way to say "I need help."
* Я нуждаюсь в помощи. (Ya nuzhdayus' v pomoshchi.) - This is a more formal and literary way to say "I need help."
Informal:
* Помогите мне, пожалуйста! (Pomogite mne, pozhaluysta!) - This is a polite and common way to ask for help, meaning "Please help me!"
* Мне нужна помощь с ... (Mne nuzhna pomoshch' s ...) - This means "I need help with..." and you can fill in the blank with what you need help with.
* Можешь помочь? (Mozhesh' pomoch?) - This is a casual way to ask someone if they can help, meaning "Can you help?"
* Я не справляюсь. (Ya ne spravlyayus') - This means "I can't handle it."
The best way to choose the right phrase depends on the situation and your relationship with the person you're asking.