To be fair, w3bbo has a point but here's how I see it..
You could jump in down webforms route.. which tries to make web development like winforms development.. and essentially makes a lot of subtle mess..
Over time you most likely will start to feel the constraints and 'ugly parts' of webforms.. but note that webforms is a very capable platform.. just not nec the most true to the vision of the web..
If you want to then take a route down a more true web route.. then you may go for an MVC/REST pattern in your applications.. in which case what you learnt for webforms will need to be 'unlearnt' to ensure you don't corrupt the concept of said paradigm..
The deal is..
You either go for webforms.. and it'll take some time to get used to vs winforms..
Or
You start learning more true web concepts.. like MVC's and RESTful design.. but it'll take you a lot longer to get the same production rate as you would with webforms..
That said.. it might just 'click' and you'll have the best from both worlds.. rapid development of well defined web applications..
For each of these you should consult:
http://www.asp.net/As this site is dedicated to the web side of .NET