2 of the main companies are FMP(Find My Past), and Ancestry. Neither of them has everything. It's a matter of where you are looking into where you think your Ancestors originate from. Don't sign up, unless you're satisfied yourself with what they offer.
There a list of free sites to look at. LancsOPC, FamilySearch. FreeBMD, GRO(General Registry Office)......and others.
Look at the free offers, for special deals that the 2 main companies are offering. I think Ancestry offer a 2 week freebie, once you've signed, and given them your Debit/Credit card info. If you go for that, you can always cancel before the 2 weeks are up. If you don't, you will automatically get signed up, if you don't cancel ahead of the 2 weeks expiry date.
DON'T BE TEMPTED BY DNA OFFERS. At least until you're well into doing your family tree. There are no shortcuts, and it's never a case of just...... 'Typing in your name, and see where it takes you'.
The most important thing is to make sure you a have enough correct certificates to start on the right footing. Like your parents marriage/birth/death certs, (if applicable). Then you need to find your Grandparents of both you Ma & Pa. If you haven't got those, then your heading for a potential disaster, of going the wrong route to your Ancestry.
General rule of thumb is 3 pieces of certified information
of all your relatives. You can never have have too much.
Most of all, talk to your oldest relatives, and see what they might know. Sometimes you may never get the truth, or perhaps it's their best guess from memory. There maybe skeletons in the closet they don't want to talk about. Can of worms or Pandora's box. You never know.
Good luck and good hunting.