Yes, it's usually best to start with the beginning of the first chapter -- that way we don't need to have anything explained, and we're coming to it cold just as a normal reader would. After a few days, once you've had some feedback on that opening, you can then use what you've learned to make any necessary changes and you can put the next tranche of the chapter up, always limited to the 1500 words for the thread. (Alternatively, some members revise the opening, if wholesale changes are recommended, and put the revised version up to check they're heading in the right direction before continuing with the next bit of the chapter.)
Then, after a week or so, to give people time to critique the second part, and again taking account of any comments, you can put another 1500 up in a new, separate thread. At that point, you can ask if anyone is willing to look at the rest of the chapter and/or the next couple of chapters for you. I'd suggest leaving it till then because people will want to see if they like your work enough to spend time on it, and also it helps to see how critiquees respond to critiques -- both as to the attitude taken (I know you'll be thanking people for their help but we do get some newbies who throw their toys out of the pram!) and also as to whether notice is taken of advice (and again I'm sure this won't apply to you, but it is dispiriting to spend considerable time helping someone only to see they've ignored all one's comments and continue to make the same mistakes).
Meanwhile, also do some critiquing of your own, so when you ask for help you can offer to reciprocate, and other members can see how useful your feedback is.