Invest in some decent kit. A lot of people look on snorkelling/freediving as the poor cousin of scuba and that you can use any old cheap tat, which is, sadly, bollocks. If you do, you'll go out a couple of times and think sod this, I'd be better off in the pub.
First of all get a proper suit, not a diving wetsuit nor a general watersports suit, but one made for freediving or, better, spearfishing. These suits are generally two piece suits and unlined with just the smooth, open cell neoprene finish. The reason for this is that it keeps water to a minimum -- harder to put on but you stay a hell of a lot warmer. The jacket should have a built in hood (again to cut down water penetration) and no zip: this is very important, a zip lets in a tremendous amount of water. The jacket pulls on over your head like a sweater... easy enough when you get used to it. The trousers, go for high waist trousers rather than a long john type. Two reasons, firstly it gives better flexibility for breathing up and secondly if you need the bog the you can just pull the jacket up and the trousers down without having to take the whole jacket off as you would with a long john. Freediving suits usually have smoothskin outers, less drag underwater and less evaporation to chill you on the surface. However if you are snorkelling regularly off the shore then you will wreck the suit (one nick and you can get a huge split in a smoothskin suit). That's why you should go for a spearfishing suit, they usually have a tough coating and will reinforcement to the arse and knees to allow you to clamber over rocks. Most suits are anatomically cut, i.e. they are shaped to follow the body's contours (even better if you get made to measure) rather than the usual flat cut wetsuits. Spearos tend to be tight gits so spearfishing suits are generally fairly cheap compared to other watersports suits when you see what you get.
I have a 5mm suit by Imersion, a French company with titanium coating on an open cell inner. It's very warm, I regularly use it over here in Dorothea Quarry which is around 7degC. Have a look at Apnea
www.apnea.co.uk they have very good prices. If you give them a call then speak to Neil, he is very helpful. I wear kayaking gloves which are quite thin neoprene. I don't use boots, instead I wear 3mm neoprene socks. Full foot fins are better than open heel fins for freediving & snorkelling and boots won't fit into the fins but socks will. In the sea I need about 8kg of lead (I'm 5'11" and 12st-8).
It's worth investing in a pair of decent fins as well, they cut down on how much effort you will expend. Freediving fins are longer and thinner than regular fins. I have a pair of Cressi Gara 3000's (about £60) but find them quite stiff. My girlfriend has a pair of Omer Bat fins, these are really nice, very, very flexible and lovely to use... but they come at a price, around £140. Omer do cheaper fins though that are still good and if you break a blade then you can replace it without having to buy a whole new fin. You can't do that with the Cressis. If you don't want to invest in proper fins then Mares Triavanti full foot fins are quite good and not too pricey.
Mask... go for a very low volume mask as it squeezes less and doesn't waste air equalising it. Again Omer do very nice masks but I use an Aquasphere Sphera wraparound mask (about £20).
Go for a very simple, cheap snorkel, just a tube and mouthpiece. Don't buy one with fancy check valves or dump valves, or a flexible barrel. Never attach the snorkel to your mask, it causes drag, can dislodge the mask or just make it leak. And the golden rule, never, ever dive with a snorkel actually in your mouth. This is how most diving agencies will teach you to snorkel but it is incredibly dangerous. All that is keeping water out of your lungs is the epiglottis at the back of the mouth if you have the snorkel in underwater. If you pass out (entirely possible even on shallow dives) then water rushes straight into the lungs and you die. If you dive deep enough then water pressure can force water past the epiglottis too (or if it relaxes for any reason). When you surface with a snorkel in your mouth then it is full of water. Blast clearing when you have low oxygen/high CO2 in your blood can make you lose conciousness. Displacement clearing (where you exhale on the way up) leaves you out of breath if there is a problem when you hit the surface. Only ever use the snorkel for breathing at the surface, if you make a dive just hold it in your hand. On the surface just let it hang loosely and it will sit in the right position (this is why you shouldn't use a snorkel with a flexible barrel, it flops into the water).
Lastly, buy a marker float and some line with a weight on the end. Apart from the obvious (letting other people know you're there) it also lets you rest if you have to.
It sounds like quite a daunting investment but if you want to enjoy snorkelling and freediving in this country then it really is worth it. In places with a good rocky shoreline you can spend hours playing about and having the right kit makes it worthwhile.
Cheers,
Stuart