Value/Where to sell my old Lego collection
Generally, I see LEGO collections like this sold by the pound.  I don't generally pay more than $8/lb (including shipping, if applicable) on large used lots, because they are rarely 100% clean, like-new condition LEGO bricks.  Most collections include poor quality pieces, as well as many non-LEGO brand and non-LEGO-compatible toys that have somehow been thrown in.

If you guarantee 100% clean, like-new condition LEGO pieces, the price would go up to maybe $12/pound.  However, in my experience, very few collections that have actually been played with will meet this standard.  Play wear isn't terrible, but it's definitely not like-new.

In the US, eBay is a common way to sell.  You might also try Craigslist to avoid the shipping hassle.

Some items are more valuable and are worth selling separately. In particular, train components and licensed minifigures (like Star Wars, Spiderman, Avengers, etc.) are probably worth the time and hassle.  Other minifigures (like town, space, western, etc) I would just include with the bulk lot, as separating those out and selling them individually will be a lot of work for minimal gain.

If you really want top dollar and are willing to put your time time, the best way is to probably start a BrickLink store, and sell all the parts individually.  This might be a great opportunity to start a side business.  It will, however, require you to inventory your entire collection, as well as manage the shipping, customer challenges, etc.

Good luck with selling your collection!