I started back around 1995 or so....I thought my first batches of beer were great....... I shouldn't have thought that, but I did.
They were mediocre to bad. From time to time I had a good batch of beer I made. But nothing special. Honestly, I would say that the first 10-15 years my beer was this way when I look back on it. I would say somewhere around 2010 I kind of reinvested myself in brewing. I read A LOT, tried to learn from others, used sources on the web, learned new things or expanded on what I knew (water, sanitation procedures, fermentation, yeast starters, etc).... that is when I feel like I consistently started to make good beer.
Lots of good advice above. If I had to give one piece of advice for getting better at brewing...... Make sure you pick one beer to brew frequently - over and over. Find a tried and true recipe or kit to start with - good source of ingredients. It does not have to be every batch, but you should brew it every 2-3 times. I found repetition and tweaking small aspects of the beer (Procedure, temperature, ingredients, water, etc) to be a key element in improving.