purchasing your first condoms is a right of passage every guy needs to go through. it's one of the common denominators between gay and straight men.
I was sixteen, and took a train into New York, I was so mortified by the prospect of purchasing them in my town. Even then, at a busy Duane Reade, I turned beat red at the checkout.
first how old are you? i think it may be illegal to purchase condoms if you're under a certain age in some places.
i think it's only been a decade or so since condoms were available at all in Ireland -- the number one piece of contraband smuggled into that once holy Catholic country. How quickly that's turned around.
Returning to the question at hand, the late ubermensch Norman Mailer once famously said 'a condom takes 50% off a fuck'. However, there's a whole new generation of condoms focusing on enhancing male/inserting partner pleasure. Some have bumps on the inside, for example. I've found the spiral ones are pretty good, but pricey.
Unless you're hung like a horse, I'd avoid getting 'large' condoms, as they're like wearing a plastic bag for those of us in the 6-7 range. There is also the very real danger of them falling off, due to not fitting snuggly. If your on the small side, 'contoured fit' is the code word for small.
However, if your planning to be on the receiving end, it's probably a good idea to have some large condoms on hand in the event you hook up with some guy with a donkey dong (btw, I wouldn't recommend this for your first time).
Of course for us guys, condoms are only half of the equation, as we don't self lubricate like women. You'll need to get some condom-compatable lube. Avoid KY, as it gums up, as do most water based lubes (including spit.) Although pricey, silicone based lubes are worth the investment. They may be more cost effective in the long run, as a little goes a long way.
lastly, I'd recommend taking some time getting to 'the main event'. don't just stick it in, or let him stick it in, as without proper preparation, it's gonna hurt like hell (btw, my first time experience!) you should definitely invest a lot of time in fingering/getting fingered, a knuckle at a time, first one finger, then two, with well-lubed fingers. This not only prepares the anus to accept the penis, it introduces lube into the it, too.
Also, I find that missionary, with legs pushed back as far as possible, is the best for opening the anus sufficiently to receive a cock. Once it's in and feels comfortable, you can experiment with different positions.