Question
I like to know how much can I charge someone to get rid of tall grass and weeds and cement a back yard, 20'x 24'  

Answer
Costs are VERY regional...especially concrete!
For our neck of the woods, I'd estimate that a 480 square foot job would be something like this:

Clear & Grub - $600
Concrete, installed - $2,400
Extras - Time & Materials (T&M)
(B this, I mean, what if they want a fountain installed, or color in the concrete, or river rocks placed, or...)

Where I got these numbers; we work with the larger contractors in our area who generally have idle crews, bobcats, haul trucks, etc.  (I'm sure some people in other parts of the country are going to read this and think, "He's out of his mind - 3-grand?!!")

IF it were just you and a wheelbarrow, I'd ask for more, based on an estimated hourly rate. (My guess is, I wouldn't do it for less than $40/hour, so one day's work would be about $320, and this yard size for one or two guys may take you about TWO days)....unless you are stong AND fast, or there are not a lot of weeds.

Concrete, for us, this goes for about $4 to $5/square foot, installed - that's with the formwork and a light broom finish.  Of course, if it isn't a simple square or has a fancier finish, it's going to be more.

PLEASE, check your local prices for concrete, formwork, etc., and don't just slap MY estimate onto your work-order.  It also depends on how hungry you are...prices are always negotiable.  For all I know, your brother owns the concrete plant. (and he may charge you DOUBLE because you are "family"!!)

There are lots of cost-estimating books from Means and others.  Just do a quick web search on this topic. Our office has the luxury of having previously-bid jobs that we designed as a point of reference for local costs.  

By the way: I've been told by our insurance attorneys that we are supposed to call it an "Opinion of Probable Cost" instead of a "cost estimate" as lots of people have been sued over this, and the courts of our land say that estimates imply accuracy (therefore, liability), whereas anybody can hold an "opinion" that is inaccurate - SILLY, but unfortunately, neccessary!

Good luck!