My advise
1) look through as many scopes at an FT club as you can
2) talk to the owners
3) create a shortlist
4) search here and the rest of the net...a lot of scopes have variants that although are subtle details can make a difference.
5) think more
6) then try and spend as much as you can...a good scope is just as, if not more, important than a good gun.
7) try and buy a good one second hand...but unless you really know what your buying dont do it blind...saves a bit.
Sidewheel + mildots are a good easier start for the beginner in my opinion...but you should buy a scope that has target turrets (ones that can be adjusted without unscrewing caps and show the actual numbers on the side) for when you drop mildots and use the turrets.
Buy one with high mag...the higher the mag, given a constant lens basis the better the rangefinding...the problem comes that higher mags push the limits of the lenses more and reduce the light, so scopes that have higher mags and perform well are large and more expensive, but to range find well is a serious % of what FT is about.
i'd say you would want to be looking into 20x +....the higher the mag the shallower the depth of field and therefore the easier to discern different distances as you adjust it. At 10x you could have 30-40yds in focus and your pellet could drop a lot over that...you want something thats going to at least tell you the difference between 30-35 yards. At the top end people are able to pick out individual yards at 45 yards +....your 10x mag would probably not be able to tell the difference between 40-50 yards.
If your unsure, ask...keep asking. I'm on my 3rd scope in a year and still looking for the next that could be better. It gets easier to pick out the next one, but it gets more expensive
Because of the changing market it tends to be that earlier models have better jap optics than latter chinese/far east variants.
A short list...
£150 mark...AGS or Nikko platinum...not great but can do a job for until you learn the other aspects of FT.
£300 2nd hand big nikko 10-50x60 or deben 10-50x56 (early ones)...18-35x40 leupolds perhaps?
£600 + leupolds 20-50x50
Hope that helps
I'm sure someone else more experienced will chime in will some help.
Theres lots to learn about every scope, so do your homework. Remember, the lower end scopes will probably not keep their value as well as a 2nd hand high end...