Teaching yourself Japanese from scratch from manga? Not the best of ideas, to say the least, but I'll go through what you're asking as best I can ~~
The first line that we're talking about is おっと、バトルを始める前に, "Otto, batoru wo hajimeru mae ni". (Just so you know, I generally romanise を as "wo" to distinguish it from お, but it's pronounced more like "o".) The "おっと" is an interjection with a kind of "oh, hold up, I just remembered something" feel to it, so I translated as "Ohh, but...". バトル is just "battle", straight from English. を is one of the most basic particles of Japanese - elements that appear after words and indicate their function in the sentence. を indicates the object of the sentence, so バトル is the object of the verb that comes afterwards, 始める, which you already figured out is "to begin". So "バトルを始める" is "to begin the battle".
To give you a little idea of how particles work, I'll just use a couple of example sentences featuring everybody's favourite simple verb, 殺す ("korosu", to kill). Since を indicates the object, Xを殺す is "to kill X". If we replace the を object particle with the が subject particle, we get Xが殺す, which means "X kills". Obviously neither of these make much sense as sentences at first glance, because in the first one you don't know who's doing the killing, and in the second one you don't know who's being killed. However, they can actually both stand alone as perfectly acceptable sentences.
The first example, Xを殺す, would most likely be translated as "I'm going to kill X". "Going to kill" because Japanese doesn't distinguish between present and future tense. And though the subject, "I", isn't stated anywhere, Japanese is big on leaving out things that are already obvious, and one of the most common things to leave out is the subject. If there's no subject to a sentence, then either the subject should be obvious from previous context, or the subject is the person who's speaking. Hence, "I".
The second example, Xが殺す, doesn't make sense out of context, because unlike in the first example, where we can assume "I" as the subject, there is no obvious object to assume. Xが殺す would only make sense in a context where it's obvious who you're talking about killing. So for instance, following the question 誰が敵を殺す？ ("Dare ga teki wo korosu?", "Who's going to kill the enemy?"), Xが殺す could be translated as "X is going to kill them." Note how the English sentence uses the pronoun "them", because "enemy" has already been stated - just plain leaving the word "enemy" out altogether is essentially the Japanese equivalent of this.
(Oh, and just so you know, there's one particle that is more common than either of these, and that's は, pronounced "wa" rather than "ha" when used as a particle. This is the "topic marker", and can generally be interpreted as similar to the subject, but it's a lot more versatile, and it can get pretty complicated explaining the differences between the two. XはYを殺す and XがYを殺す both mean "X is going to kill Y", but there are clear differences in meaning between the two, and you can't just substitute one for the other. But let's not go into that.)
Anyway, back to the original sentence, there's one part we haven't looked at, and that's the final 前に ("mae ni"). 前 means "in front" or "before", and に is another another common particle that indicates things like location and time (similar to things like English "at" and "to", if you like). So 前に, in this context, means "before", and it's being modified by the バトルを始める immediately before it, so all together it means "before (we) begin the battle". Note how the "we" doesn't appear anywhere in the sentence - it's an obvious subject that gets dropped. 僕達がバトルを始める前に ("bokutachi ga batoru wo hajimeru mae ni") with the additional "bokutachi" meaning "we", would also mean "before we begin the battle", but it would sound odd, because the 僕達が feels unnecessary; it's unnatural to put it in. In fact, putting it in makes it sound like you're emphasising the "we" for some reason, so it sounds more like "before we begin the battle..." (to be followed up by something like "...somebody else is going to interrupt and begin it instead.")
...Hope that helped ~~
