The basic basics you need to know:
0. We use the simplest and best definition of bisexual: attracted to more than one gender.
Note that there's nothing in that about being attracted equally to all genders, or there being only two, or being sexual with more than one, or not being monogamous, or being unfaithful, or…
If it's not about someone's attraction / thoughts / desire / fantasies / etc etc, but something else, like how they identify to themselves or others, or having been sexual with more than one gender, we'll say something 'bisexual identified' or 'bisexual behaviour' unless it's already really clear.
1. Similarly when you talk about sexual orientation, make it clear whether you're talking about attraction, identity, or behaviour – someone's thoughts, their words they use to describe themselves, or their sexual deeds.
2. If it's sexual identity, make it clear that this will understate the number of bisexual people, because many, many more people are bisexual in their attractions, and even their behaviour:
(Adapted from Rebecca Jones,* as republished in The Bisexuality Report)
It's also quite common for bisexual people to use other words like 'pan / pansexual', 'queer', 'bi-curious', 'biromantic', 'omnisexual', 'questioning', 'lesbian-identified bisexual'… for their identity.
3. Because bisexual covers so many people, with so many different sets of attractions, behaviours and identities, you shouldn't assume anything more than them being attracted to more than one gender.
Some bisexuals are equally attracted to all genders, for example, some aren't. They're all bisexual.
99. There's no hyphen in 'bisexual'.
It's 'homosexual' and 'heterosexual',* not 'homo-sexual' or 'hetero-sexual', so don't look silly and write 'bi-sexual'.
Repeat with 'biphobia' etc.
* Jones, R. L. (2010). Troubles with bisexuality in health and social care. In R.L. Jones, & R. Ward (Eds.) LGBT issues: Looking beyond categories (pp.42-55). Edinburgh: Dunedin Academic Press, p. 46
** Other sexualities exist. Your sexuality may change over time: past behaviour is not a guarantee of future attraction or identity or behaviour. Bisexual community-building and health promotion work is funded much less than gay or even lesbian community-building and health promotion work.