Welcome to my Brolaf guide! This is my first guide so please don't be too strict with me about typos or grammar mistakes ;) I've played Brolaf pretty often and was mostly very successful with it. I think Brolaf is a very interesting champion, who you can actually play either solotop or jungle. This guide is gonna focus on the solotop part, so if you're looking for a jungle guide you should check out a different guide on Mobafire. Feel free to comment if you disagree with me somewhere, I hope you'll enjoy this guide.
This guide is not complete yet, I'm going to update it soon.

Pros
Cons

I think that Ghost is probably the best summoner spell I can imagine with Brolaf, since there is no CC you can flash out from. Since your most important job is to rush in and kill the enemy ADC, you can use Ghost, Ragnarok and you have 6 seconds of not being CCed in which you have time to kill the ADC. But since Flash is almost always viable(flashing through walls, flashing in, flashing out) I wouldn't be mad at you if you use it....
I almost always take Ignite, since you can use it, to ensure a kill at toplane and you gain more AD thanks to Summoner's Wrath while it is on cooldown. Exhaust, Teleport and Heal are possible to, but i really do prefer Ignite.

I mostly take Greater Mark of Desolation, since Brolaf's AD scales are low (or at least your Reckless Swing, which is your main skill).
On Seals i use Greater Seal of Resiliance, increasing your survivability against Heavy AD and allowing you to survive a long time in in teamfights.
I take Greater Glyph of Magic Resist, since the damage by the enemy also consists of AP damage, you will be burst down pretty fast by the enemy Mage without those.
And last I'm taking Greater Quint of Strength. You are an AD Tank after all.

On level one im getting my Undertow, since it has much range, slows and you can pull the blue buff easily.
Next, I'm trying to max my Reckless Swing as soon as I can so im putting the next skill points in it. There are many people who skill Tough It Out at lvl 2, I usally do that only if my opponent dominates the lane in order to get more sustain.
I skill Ragnarok as soon as I possible.
I often max Tough It Out second, but when I see that my harrassing power is more important than the lifesteal and AD, I'm going for Undertow instead.

I recently even saw voyboy maxing Undertow first, because he got the blue buff donated since lvl 1 (the jungler was Mundo). This actually is a pretty dangerous playstyle and requires very good team cooperation. But if you are playing good with your team and want to try something new: Go for it.

This section is pretty long so I beg for your patience since this is a very important part of this guide.
There is a wide range of Items that are very useful with Brolaf. The ones I use the most are contained in the item built I described above.
I prefer starting with Boots and Health Potion. Some people start with Cloth Armor, which is actually also viable, but i think the boots are more effective, because you are more likely to hit the enemy with your Reckless Swing. I'm usally going back with 900 gold and picking up a Vampiric Scepter, a Cloth Armor, a sight ward and Health Potions. There are many Brolaf's, who go for double Doran's Blade. I usually take this, if the lane is not going quite in my favour. If I suck in early game, I purchase the Gold Items ( Philosopher's Stone and Heart of Gold), after I get double dorans. But please don't do the mistake and go for them when you're fed, since they actually drag you down and just prepare you for the lategame. There are even some crazy Brolaf's who actually go for double Doran's Shield. But as surprising as it might seem: I tried it out once and it actually worked pretty well.
After that, get Wriggle's Lantern and Mercury's Treads. I think that Wriggle's Lantern is a very useful item, since it gives you a free ward every 3 minutes. But if i go for Double Doran's i usally skip that Item. Only when the enemies have VERY HEAVY AD, im going for the Ninja Tabi instead of Mercury's Treadsbut that barely ever happens.
Then I'm going for Phage and later for Frozen Mallet. If you got Wriggle's Lanternearlier this is almost a must-have since it makes you so much tankier and slows the enemy. If you go for double Doran's, you have a pretty cool amount of HP already and you can maybe go for Hexdrinker first, which would be my next core item. I'm going to get Maw of Malmortius later. This one is a really awesome item on Brolaf: Berserker Rage gains you more Attackspeed the less life you have and this is going to get you more AD the less life you have. => Your overall Damage output is going to EXPLODE whenn your health drops. Besides, it gives you a nice amount of Magic Resistance and Atack Damage.
If I didn't get Wriggle's Lantern earlier, I mostly buy a Vampiric Scepter first and build it into a Zeke's Herald for the lifesteal, health and atackspeed. Furthermore the allied ADC profites from this since it has a lifesteal aura.
Now this is when it pays off if you went for the Gold Items earlier: You can built your Philosopher's Stone into a Shurelya's Battlesong and your Heart of Gold into a Randuin's Omen. Shurelya's Battlesong is a very important item for Brolaf: You can either use the speed boost to stack it with your Ghost, so your going to be instantly at the enemy's ADC or you can use it for a speed boost when your Ghost is on cooldown. Your Randuin's Omen is very usefull since it can slow your sourrounding enemies. Both of them can decide the outcome of a teamfight.
In case i didn't go for the Gold items before, I still go for Shurelya's Battlesong but as last item mostly Atma's Impaler.

A short summary in case you've already forgotten half of it:

If dominating lane:


If being dominated in lane(Gold items playstyle):
Zeke's Herald

Two Doran's Blades can always be bought in early to mid game.

Brolaf is probably one of the most aggressive Solotops in the game.
Since it is very important to get a high creep score, I usually get some last hits with my Undertow if I wouldn't get it without it. This way I can stay out of the enemy's harass range and I can poke the enemy a little bit if my Undertow hits the enemy too. Mostly I start my real harass at lvl 3, slowing the enemy with my Undertow and trying to get a Reckless Swing on him.

The most difficult job of a solotop is, to estimate the own and the enemy's damage output. The good news here is, that Brolaf just does 340 true damage with his Reckless Swing (at lvl 9). No scales. No armor calculation. Just. 340. true. damage. So it's actually way easier to estimate your damge output than with other champs.

If you get frustrated because the enemy is just tower hugging and you aren't able to get a kill, you should start thinking about a tower dive. But it's important that the conditions are almost perfect: The enemy jungler has to be far away, the creeps have to be at the enemy tower and you have to be absolutely sure that you are going to get out alive.

If that is the case, you can go for it. Try to provoke the turret aggro as late as possible. If you miss your Undertow you should retreat immediatly. If you hit with your Undertow and have already been targeted by the Turret, you should use Ignite if available. After the kill, run in the direction, in which you take the least tower hits. Now that you've accomplished a kill, you can either push down the turret or go back and take advantage of the gold lead (depending on your remaining life).