The first thing to consider is a Goal. Whether it be for overall well-being, health issues, preventative measures, or to simply tighten up that tummy for summer time. A good goal will give you the motivation to be persistent with your path, day after day thinking about it and tackling it. It's very important you are working for something you really want!
It is also important to understand what your goal is, and how realistic your goal setting has been. A "S.M.A.R.T." (Specific, Measurable, Attainable, Realistic, Time-based) goal is a great place to start. This acronym is commonly used in many workplaces and can have small variations but really has the same point: make a realistic goal that you can measure and stay accountable to.
The next thing to consider would be if you have any pre-existing injuries or limitations that may prevent you from performing certain movements with a safe and effective range of motion. There could be imbalances or injuries that you may not have been aware of which could emerge as you put your body through a more vigorous fitness routine, Good practice is always to start with a thorough assessment with a chiropractor, massage therapist, and your family doctor. This can help give you a baseline understanding of some of the issues you may expect to see pop up.
Now that you have a good understanding of the imbalances and injuries you may have, and have been cleared to start a new physical routine, you are able to start a new workout plan! This is a big step for your body, and you have many paths to choose from. Should you listen to your cousin (who swears by cardio only), or maybe it's best to just do what you remember from grade 9 gym class and buy some free-weights? So many options! The way I started was buying a few bodybuilding books and putting my own inspiration into the workouts that I saw. I spent over 3 years getting nearly no results aside from a little strength gain and a forearm injury - don't be like me. Although I was stubborn enough to spend three years going through this, I eventually hired myself a trainer (who I carefully picked) and it changed my life completely. Not all trainers are the same, but I had a great experience with mine and it inspired me to be better! Now, after spending over a decade in the fitness industry, I can confidently say it was the best choice I've made. This is not a sales pitch for personal training, I believe if training is the right way for you, you will come to the conclusion yourself (hopefully sooner than I did).
This leads into the biggest hurdle for many people who are either starting a workout routine or are getting back into fitness, which is knowing how to do exercises properly. There are thousands of exercises out there that can be combined in different ways to make an effective workout routine. That being said, at the end of the day, they can all be broken down into a few foundational movement patterns which must be learned properly, and that's ok! You can learn them at any stage, however, it really is a learned skill, and thinking (as I did) that it should be easy or just come naturally, is simply not the case. They are movement patterns created to maximize mechanical efficiency and safety, which at one point in our lives were extremely natural (if you've ever seen a toddler do a squat, it's nearly perfect - with no training). As we age, and get into more limited rages of motion and are less active. Things tighten up and we become less mobile. Our bodies do their best to compensate, but usually we are left with imperfect movement patterns that need to be corrected before safely ramping up our exercise routines. This is also one of the big reasons why every person should be doing their own individual fitness routine: we are all different!
I truly hope all this will help ensure the journey to your goal is a safe and productive one!
References:
How To Increase Serotonin in the Human Brain Without Drugs
Simon Young - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2077351/
Effects Of Physical Exercise on Cognitive Functioning and Wellbeing: Biological and Psychological Benefits
Laura Mandolesi-Arianna Polverino-Simone Montuori-Francesca Foti-Giampaolo Ferraioli-Pierpaolo Sorrentino-Giuseppe Sorrentino - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5934999/
Atherosclerotic Plaque Rupture and Thrombosis. Evolving Concepts
V Fuster-B Stein-J Ambrose-L Badimon-J Badimon-J Chesebro - https://www.ncbi.nlm.nih.gov/pubmed/2203564
Assessment and Treatment Of Endothelial Dysfunction in Humans
Todd Anderson - http://www.onlinejacc.org/content/34/3/631