Neglected overgrown garden with lots of trees?

Okay well on our farm there is this old house that the previous owners of the farm lived in before we bought it like 20/30 Years ago. There's a garden beside it and we've never really used it or taken care of it because we have our own Hugeee 3/4 acre garden but I'm off for summer and I just thought it would be brilliant to bring it back to life because it could look beautiful. There's loads and loads of trees along the two sides and some have been there for about 100 years I'd say and there's branches and weeds and nettles etc all over the ground and there's not much light getting in because the trees have come together at the top. Theres some trees that have fallen down as well. Any ideas on how I could tackle this? I can get help obviously :) could you advise me on what to do ad where to start and how to do certain things?