Start with learning Hiragana and Katakana. There are many websites and images with them (plus stroke order). Perhaps get a Memrise or Anki desk with them in it, so you can learn them quickly.
Learning the numbers 0-10 is good too, as it helps with learning 11-100 and up. This also helps with telling time.
Next, basic sentence structures with present and non-present tense in positive and negative. The verb endings change in a predictable pattern for a beginner. There are two irregular verbs to look out for in the beginning. To do > suru, and To come > kuru.
Here are some examples of sentences to learn (present tense with some negatives):
Hello (morning/daytime/evening/night) Ohayou gozaimasu (polite) Ohayou (casual), Kon'nichiwa, Konbanwa, Oyasumi nasai.
Goodbye. Ja mata, Sayounara (if you won't be seeing them again that day)
My name is ... / What is your name? Watashi no namae wa [name] desu. or [name] desu. / O-namae wa nan desu ka?
It's nice to meet you. Hajimemashite (at the start of a conversation with a new person)
It's nice to meet you. Yoroshiku onegai shimasu (at the end of the introduction with your name)
How are you? / I'm good. O-genki desu ka? / O-kage sama de.
Thank you. Arigatou gozaimasu (polite). Arigatou (polite). Doumo (casual)
I'm sorry. Gomen nasai (polite). Gomen (less polite but okay). Sumimasen (polite)
Excuse me. Sumimasen
I don't understand / I understand. Wakarimasen / Wakarimasu.
I can speak [language. Watashi wa [language] ga hanasemasu. (put eigo for English, nihongo for Japanese)
I can speak a little Japanese. Watashi wa sukoshi nihongo ga hanasemasu.
Can you speak [language]? Watashi wa [language] ga hanasemasu ka?
Yes / No. Hai / Iie
Remember once you've started speaking and the topic is about yourself, you can omit watashi.
Also remember, you don't pronounce the u at teh end of desu, masu etc
1. Learn Hiragana and Katakana
2. Learn the japanese particles
3. Learn nouns
4. Learn verbs(I recommend starting with the -masu form of the verb, learn past and present/future tense)
5. Learn about the -i and -na adjectives
6. Learn grammar then try creating simple sentences in hiragana/katakana using the particles and vocabulary you learned.
7. Have someone check your sentences.
For example, today I will learn about the は "wa" particle.
Then I will also learn about the pronoun 私 "watashi" which means "I" and then the verb です "desu" which means "to be".
So now I learn this grammar ◯◯は◯◯です "◯◯ is ◯◯"
Now I create my sentence "Watashi wa Marie desu" which now translate to "I am Marie".
Now try the grammar again with a new word これ "kore" which means "this" so you get something like これはCakeです。 "Kore wa cake desu" which means "This is a cake".
Repeat with diffirent particles, nouns, adjectives and forms of verbs until you can express what you want to say. Then always have someone to check your sentences so that your sentences are correct and so that you won't be developing bad language habits in the future.