Trading firm does not raise capital from third parties.

The owners of the firm usually put their personal money in. They hire traders, some of them seniors, some of them juniors. Seniors with good track record sign commercial agreements with the owners. Terms are all case by case.Salary+part of the profits are typical compensation package. If they loss money, they may get fired but I don't think they have to make up the firm's loss.

From the tone you ask the question, I am pretty sure you are new to that segment of the industry. If you are beginner, you probably can only be a day trader. It is likely you won't get base salary but a percentage of the profits you make, of course the firm allocates some capital to you.

Good luck.