While basic servicing on your car can be done on your own, engine repair and transmission repair should be done by a professional. If someone with little or no experience attempts a major repair, more damage can be created, and then it needs to be repaired by a professional, and the mechanic will charge accordingly.
In most cases, if a mechanic has to repair a “botched” job, the cost will most likely run higher than the original repair if it had been taken to the shop in the first place. If the job seems too big, it probably is.
Any safety related repair should always be done by a professional, your safety and the safety of others is at stake. You can incorrectly do a tune-up and your car might not start, that is bad enough. Do your own brake job and your car might not stop. This can have tragic results.
Front end work, like tie-rods or ball joints, is another safety related job that should not be attempted on your own. If you want to be able to do this type of service to your car make sure you learn the proper way to do it.
Oil or fluid changes can be done yourself, also belts and hoses, these don't require a lot of expertise, but you still need to be very careful and follow directions of course. Tire rotations, changing blown fuses and tune-ups with a little practice can be quite easy or even fun to do on your own, so don't hold back.
Just know when to say "I think this job is too big for me".