If you are interested in becoming a mechanic, then it is important that you understand the field that you intend to join before making a final decision. This can be done by talking to the people who are already in the field and know all that there is to know. An even better idea is that after talking to them you take up an apprenticeship of some sort with a mechanic to get hands on experience and see if it works for you.
If it does then you are good to go and start looking for ways in which you can get a formal training in the field. If not then this would be a good point to back out and look for an alternative. Backing out after most of the formal training is done and most of the cost is covered is not a really good idea.
Good apprenticeships are not difficult to find. This is especially so when you agree to work for free. If you make it a point that you want to earn through this internship then it may be much more difficult to find work. Someone who is totally new to the field and will need everything to be explained to him first and who also demands money is not going to make a very good candidate.
Some might think that working for free like this is an injustice but this is not the case. In order to be mechanic you need to make sure that you are going to get the maximum benefit out of all of this. Even if one intends to back out after this point, the result is not going to be that bad. At least you will have the basic knowledge with you, which can come in handy at any point in your life in the future, no matter in how small a manner.
Giving up on some of the money here can be treated as the fee that one might be paying to some institution for formal education. This expense is generally considered as an investment in the long term and for a better future. The same rules apply here. Once you start looking at the situation in this manner, then all of this is not going to look like a burden or injustice. A positive attitude matters.
You should start looking for an apprenticeship as a mechanic as early as possible in order to secure a spot in a good program.