As an adult beginner who started cello a little over a year ago, I have the following advice:
1) Find a teacher. You can learn music theory through the Internet or a book, and you can do ear-training exercises through software. However, you cannot learn to play a string instrument with any reasonable level of proficiency without taking lessons.
2) Take the time to find the best teacher you can. Some teachers will have good personality. Other teachers will be convenient. Neither means they're a good teacher. Look for someone with a proper pedigree. That means they went to school for your instrument, and preferably have an advanced degree. Also they should perform regularly on your instrument. That means they play in a professional orchestra or a high-end community orchestra or do studio work, or play in church, etc. Once you've assembled a couple people who meet these criteria, take a sample lesson from each one and see which one fits you the best.
This is not to say there aren't people who lack formal training but happen to be great teachers. I'm sure they exist. It also doesn't mean that someone with great credentials can't be a terrible teacher. However, your chances of finding a teacher that really knows their stuff increases dramatically if they've had to go the years and years of training necessary to get a degree.
Logistics may dictate that you take someone not at the top of the list. However, be sure you really have no other choice. A year of lessons from a top-flight teacher is worth three years of lessons from a mediocre teacher. If you have to drive an hour to get to the top-flight teacher, it's still worth it.
3) From the beginning, resist the urge to mark your progression by how many Suzuki books (or any other kind) you've completed. Mark your progression by how well you're mastering intonation, rhythms, counting and bow strokes. In other words, the fundamentals. If you try to move too fast, you'll eventually get to the point where the material is too difficult for you, and you'll be stuck trying to fix the fundamentals before you can move forward.
4) Do not watch tutorial videos on the Internet unless you've discussed them with your instructor. The Internet is full of videos from "experts" with HORRIBLE technique. There are some good ones out there which are fine, but you have to learn which ones are good and which ones are bad.
5) Be patient. String instruments are not easy. But the accomplishment of becoming proficient on one is something of which you will one day be extremely proud.