Re: Starting out at the age of 25
As a beginner, you could start by doing jab-only drills. It's an awesome way to sharpen your jab while also getting used to defending against them. The way to do them is to take turns with one only throwing jabs while the other only practicing defensive manuevers (blocking, parrying, slipping) and footwork. Then switch roles every 30 seconds for 3-5 rounds all together. It's a defensive and offensive drill all in one.
Also, don't get impatient and go into all-out sparring when you don't have a clue on what to do. It just seems counter-productive. An experienced and competent trainer normally goes through all that with you before even letting you spar. Without a trainer or even another trained boxer to work with you, it'll be hard to learn things without picking up bad habits in the process.