A common beginner mistake is over ambition when starting an army collection. Ambition and big dreams are good, but when that creative energy is unfocused it often leads to burnout and unfinished armies. When starting an army, several steps can be taken to avoid a loss of desire and ensure you get over the line with a finished collection.
By this stage you have already chosen a faction, which is great. A lot of dreaming over army possibilities and paint schemes has happened, but now those dreams need to be focused. To channel the creative thinking, you first need to settle on a paint scheme. To save money while developing a template, choosing a set like this one from GW is a great idea. Cheap with no gluing required for these marines, you can practice to get the exact color you want. For example, with my Genesis marines it is not enough to decide I want a red and gold scheme. Do I prime the models black or white? Do I use Mephiston Red or Khorne Red? What shade of gold do I want? Experiment with different shades, tones, and base coats. Once you get a scheme nailed down in terms of exact colors and painting techniques, then you can start buying bigger units to assemble your army.
If you just start getting the exact units you want in your army without testing out your paint scheme first, you may be frustrated if the colors don't turn out the way you expected or wanted. Now you are stuck with your money tied into a half finished unit. Patience is the key. Find the exact scheme you want before assembling your true force.
Once you have settled on a paint scheme, and after you stock up on paint, now comes the lengthy but exciting process of assembling your army. I am assuming you already have an army list drawn up, so I won't discuss that phase of planning. I will say though, plan on a small army, 1,000 points at the most. 2,000 plus points armies are fun to think about but take a substantial amount of work and time. Don't discourage yourself with too much work.
When you start the process of building your army, I encourage you to buy and paint one unit at a time. GW offers a lot of bulk sets now, which do save some money on the front end, especially with the advent of unbound army lists in 7th edition, but those bulk sets have dozens of models. Often when just starting out your excitement will lead you to purchase several units if you have the money. Maybe 2 tactical squads and a Rhino or a Tau Fire Warrior squad with some Pathfinders and a team of battlesuits. Whatever the case, having 20, 30, and sometimes 40 or more models waiting to be finished will be discouraging. Assembling a horde army can quickly become a daunting task. The goal of finishing seems unreachable, and often the end result is either a boring, unpainted army or model boxes collecting dust in the attic and the world of 40k loses another player.
Building one unit at a time presents a reachable goal. Ten space marines is not as intimidating as 25. You'll be able to see tangible progress and feel good about completing a unit, which will fuel your efforts to complete your army. Finish a unit completely and then purchase another.
Start small, but always dream big. One day you can have a large, fully painted force, or maybe even several forces. Don't be afraid to dream but channel those energies. One patient step at a time is the best way to ensure success. Don't give up, and just keep painting!