you haven't said what country you are in - different laws will apply in different countries. i'm no expert on the law, not even for my own country (the UK), but i'll try to give advice as best i can.
what yahoo and thousands of other websites throughout the world do ("I accept your Terms") is basically common practise. sometime's it's done by checkbox, sometimes by radio button, sometimes simply with text that reads "by proceeding, you are agreeing to our terms as shown <here>" etc. regardless of any legal obligation to make a customer aware of your terms, and regardless of any legal requirements for *how* to do this, and regardless of the legal validity of doing this, it's always a good idea. it looks more professional and can prevent the odd ordering problem - ie, ensures the customer is well aware of the services you do *and do not* provide.
countries within the EU should by now have updated their laws that state that contracts concluded over the internet (and by other distance selling means) to ensure they are legally binding or as legally binding as possible (read on).
as far as i know (and don't quote me on this), UK law allows any agreement, whether verbal or written, signed or not, to be legally binding. but in the event that there is a dispute that goes to court, if there is no proof that such a contract ever existed, the case could be dismissed. any case in court would depend on both parties agreeing that such an agreement ever existed, and any dispute would therefore be about the terms of the agreement. i believe that contract laws in most other countries are based on similar "burden of proof".
the UK has the Consumer Protection (Distance Selling) Regulations 2000 which set out legal requirements for information to be displayed on websites - ie, that terms must clearly be displayed prior to the customer placing an order from a website etc etc. this law sort of implies that any online ordering involves "a contract" - there is nothing that explicitly states that when an order is placed online, there is a legally binding contract (but read on).
the UK E-Commerce Regulations (from my brief reading so far) explicitly state that when orders are placed by distance means (ie, online ordering), there is a contract between the customer and the site owner (company / retailer / service provider or whatever) and just like any other contract, this contract *is* legally binding.
there is however, one slight problem - proof. there is nothing to stop me changing my terms once a purchase has been made, posting my new terms to my website, then telling the customer i don't provide the service he paid for. the UK government has sort of acknowledged this as a problem but has deliberately and quite openly avoided finding a solution - it looks like they prefer to let the courts set a precedent and amend the law later.
to explain this a bit more, the documents state that in certain cases, contracts should be provided by "durable medium" but they stop short of stating precisely what is and is not a durable medium. in general terms, we could expect a "durable medium" to mean something like "in a form that allows the customer to retain a permanent printed copy" of the terms in force at the time the transaction was made. the government suggest that fax or email could be "durable mediums", but they only *suggest* this and do not state that they *are* durable mediums. they do not suggest web pages could be a "durable medium" - there is no indication as to whether this is a deliberate or accidental omission. in contrast, the german government has stated that web pages are a durable medium but that emails are not.
at the end of the day, fax, web pages and emails can all be faked, and until such time as there is a simple means of proving the terms in force at the time these contracts were made, the law will remain vague.
my advice for a quick and cheap solution without needing to employ lawyers etc - look at several major companies in your country to see what they do and how they do it, and if you do something similar, you should be ok. they won't want the bad publicity associated with operating "in breach of" the law, so they'll generally keep on top of it and keep it legal. whether or not such contracts are legally binding will depend on the country / state you are in and the local laws there. if in doubt, consult a lawyer.
once you've set up your site with terms and online ordering etc, you still have the problem of enforcing contracts - if someone reneges on their contract to pay you for your services, you have to decide whether or not to try and enforce the contract and obtain payment. sometimes it just isn't worth the effort. i'll leave the final decision on that up to you!