It depends what you are trying to do, but an existing network will always have more people power and more reach than an individual will. If you can find groups in your area who are campaigning on issues you care about then you should try to get involved and just see if it is for you.
You only really need to establish something new if what is already out there doesn’t align with your values or the kind of activism you want to be engaged in. The bigger organisations often have a lot of ‘baggage’ and internal conflicts, or a certain way of doing things that they’ve become accustomed to. That happens with any organisation over time though.
That said, there is still plenty you can do without being part of an organisation. I had issues with some of the groups in my area when I first started out so I went and did my own thing, which suits my disposition, my skillset and what I want to focus on. That wont work for everyone, but lots of individuals working towards the same cause in different ways can achieve just as much as a centralised organisation can.