If you aren't using a character sheet, you have very basic options. The Attributes and Abilities tab is divided into to two panes. The Attribute pane holds whatever you want to define as attributes, this can be character attributes (STR, DEX, etc), weapon attributes (broadsword_damage, broadsword_attack, etc), and any other talents and skills you need. The Abilities pane is used to create dice rolls, and can draw on stuff from the attribute pane. For instance a weapon attack might have the text /roll 1d20+@{DEX_Bonus}+@{Broadsword_Attack}
Damage: [[ @{Broadsword_Damage}+@{STR_Bonus}]] In the above example everything between @{ } is an Attribute, defined on the left pane. Or you might define everything about the weapon in an Ability, like so: /me attacks with +2 Broadsword.
/roll 1d20+@{DEX_Bonus}+2
Damage: [[1d8+2+@{STR_Bonus}]] This is probably easier, and is the only way you have to define weapons in roll20 without a Character Sheet. You can quickly guess that recording a character with many different weapons, skills, stats, and so on, would be very laborious and pretty ugly, but it's doable. It's what we had to use before they introduced the Character Sheet feature, which wasnt part of roll20 initially.