Why do you play Magic at FNM? Well, the biggest reasons are because you enjoy playing, to be social, or to win. Maybe a mix, or all of them. If you play to win, it's because winning is fun, hence you play Magic for fun. If you play because you enjoy playing, then that is your fun. If you play to be social, sideboards may not be much to you, however they can still be nothing but beneficial.
Play to win. If you play Magic at FNM because you enjoy winning (and lets face it, we all enjoy winning, it's just naturally enjoyable), then this should be fairly brief, because you probably already use a sideboard. If you don't, then you're missing out on big chances to beat decks that you normally couldn't. Basically, you need a sideboard to be able to adapt to decks that you normally can't and in doing so have a better chance of winning. As another note, nobody plays FNM for the prize, they play for winning. Getting the prize is just some nice trophy of your deed. If someone actually wanted to be making money or gaining from this, they'd be the one buying Innistrad booster boxes immediately after rotation, to sell for five times the price in a few years. Chances are they don't even turn up at FNM.
Play to be social. After finishing the first game, if one player has a sideboard, but the other doesn't, it can sometimes make the player with the sideboard feel awkward or mean for using the sideboard (try not to feel that way, though, it's a part of the game, just optional), whilst their opponent doesn't. It also leaves one player waiting. If both have a sideboard, they can both be getting to it, not feeling awkward, and you can even have a chat that doesn't start with "You don't have a sideboard?" Additionally, sideboards have no negative effect, at the worst it simply does nothing, so skipping out on the chance to one-up an opponent is simply not good logic.
Play to play the game. If you play because you just enjoy playing, then you may not see a need for sideboards - a deck should be able to perform enjoyably all the time, correct? Well, not really. Sideboards are just another part of the game, and they can really help you enjoy your games more. For example, lets say you play whatever deck you have fun playing, but end up against an opponent with a deck that stops yours doing just about anything, and you end up just watching as he plays his game, whilst you draw a card a turn and watch it die. For most people (except the opponent, likely), that's not fun. It's also not fun to suddenly die because your opponent played two cards that combine to kill you. So, for the next game against him, rather than give up hope, you can use a sideboard, to bring in other cards that you enjoy using but also will help you avoid the unfun elements of the last game. For example, you could bring in a card that can't be countered and start ripping up that "Control" player, or bring in a card that can destroy enchantments to be able to stop someone winning out of nowhere with their "Combo".
Onto some tips for helping with your sideboard. I'll present these in bullet point form, as if you want more help there are tonnes of articles out there that will go into depth.
Firstly, if you didn't want to read the above, some summary points to why you should use a sideboard:
-At the least, they do nothing, i.e. a sideboard will never hinder you, it'll only help.
-Sideboards are a part of the game, and you have every right to use one.
-It can be awkward if only one player starts sideboarding.
-You can stop people messing with your fun.
-You can get a good advantage against certain kinds of decks.
Some tips on what kind of cards you should try to fit into your sideboard:
-If you're using an aggressive deck, try to have something in case you face a more aggressive deck. And don't assume yours is the faster there is. It's not.
-If you're using a deck based around a combo, or synergy, have something in your sideboard that can work as a plan B, that isn't weak to what the combo/synergy is weak to.
-If you're using a controlling deck, have cards in your sideboard to be able to replace "dead" cards in your maindeck, e.g. removal spells when against a deck with few creatures.
-Take a bit of time (or testing) to try to figure out what kinds of cards beat your deck (e.g. lifegain if you're aggro, or what stops your combo), and have something in your sideboard that can counteract those cards.
-If there are any prominent deck types (e.g. "Bant Hexproof Auras") at your FNM, try to include cards that will be beneficial against those decks.
-Include cards that can be used in your deck easily (e.g. fit in with your Vampire theme), in case you need to take out dead cards.
-It may not be a bad idea to include a land (basic or non) in your sideboard, in case your deck isn't tested and you're worried about getting "mana screwed".
And finally, some tips on general sideboarding:
-Although you're now allowed anywhere between 0-15 cards, try to use 15, because it doesn't hurt to have more options.
-Similarly, you're now allowed to sideboard as many cards as you want, so long as you still have at least 60 in the deck. Try to keep yourself at the same number of cards, as the more cards you sideboard in, the less streamlined your deck becomes and also the land count will be skewed.
-When sideboarding, you do not have to say how many cards you've sideboarded, whether you've changed 15 cards from your deck, or simply added 1 card. Your opponent is still allowed to count your deck, though.
-Only sideboard out lands if you're also "siding" out the more expensive cards in your deck. Even then, try not to take out any more than 2 lands. If anything, try siding in lands.
-Be aware that your opponent could be changing his deck in foresight for how you're changing yours.
-Finally, ask for help. Just about everyone will be willing to help you with your sideboard.
A quick disclaimer: This is mostly just personal advice, and should be taken as such.
Nathan, Gambit Games USA