Well, its a whole can of worms really. It often centers around the possible future value any ownership may represent.
Generally, depending on the stage of the startup and the funding available, you may be asked to trade off your wage earning value for some level of ownership.
In return for taking this risk you may get a shot at the brass ring. However, if things don’t work out, many startup companies fold, make sure you aren’t out on a limb.
If you are young and know that the job market is good, then you can afford to take a shot in a startup. The smaller the startup the more issues you will be faced with, whether you are ready or not to deal with them. You may accrue a lot of responsibility very quickly and deal with a huge variety of issues you might never experience in a larger company.
These things can be valuable even if you are never compensated fully for the time you put in compared to a more established placement. It can even be hard to downward adjust to a real job later on.
Anyway, before jumping into a startup try to get a handle on some things. The smaller the company or earlier in its life the more important this is.
Does the company have a realistic plan?
Do they show revenue and sales yet? Are profitability projections reasonable?
How dependent on investment is the company with respect to its goals?
Does the company have capable people in important roles?
How much does the company rely on your contribution in order for it to succeed?
Are you going to have the budget and the personnel to achieve the goals outlined for you?
Are there well defined goals?
How are they going to measure the performance of yourself and others?
Are the people judging your performance realistically aware of appropriate timelines, costs and methodologies?
Does the company have a good understanding of its market and its competitors?
Is the life of the company tied to an initial customer or initial contract?
Is the market the company will be in regulated?
The answers things like this may help you figure out what you are willing to accept. The better the answers the more you may be willing to concede to get your foot in the door. Anyway, not trying to scare you away from it, but make sure you go in with your eyes open. It can be great experience and if you go in, try to make sure it is structured so that it will be.
Speaking from experience, having junior people without qualifications making decisions that affect your productivity can be troubling. Make sure you are given the authority and means to accomplish your responsibilities.
Anyhow, do some Googling, I’m sure there are online negotiation guides out there if you are down to the finer points already.