Cash is one of the most important resources in a startup, which makes fundraising critical for your startup's success. There is a lot of excellent advice on getting funding on the web so I will only cover the basics here. 
Capital directly determines the runway that your startup has and so the most important thing to keep in mind is that you don't want to run out of cash. Naturally, the cost of experimentation varies dramatically among different industries, which will determine your cash needs. In any case, the more proof you have that your hypotheses withstand contact with the market, the more appealing your startup will be to investors. That is why it is crucial to design a solid fundraising strategy based around milestones that focus on testing assumptions while minimizing dilution. 
In addition to progress, investors will be evaluating your market, team, and product - usually in that order. This is because the market you are in and the composition of your team determines the potential of your venture, while your product will almost certainly change based on customer feedback. Professional early stage investors are well aware of the fact that most startups fail, which is why they seek ventures that have a chance of making outsized returns. That is why, as Peter Wendell from Sierra Capital often says, investors' greed needs to overcome their fear for them to make an investment.
A common source of tension between investors and entrepreneurs is the fact that investors can diversify risk by backing multiple startups, while entrepreneurs don't have that luxury. Furthermore, early stage investing is not just a transaction but rather a permanent marriage, which is why it is crucial to spend time researching investors beforehand. There is no excuse for not doing your homework on investors because there are bad investors just as there is bad everything. Finding an investor that fits well with your vision, values, and goals will help avoid most problems that can be avoided.
Finally, investors can add a lot more value than just cash. Great investors will help with recruiting, business development, strategic advice, and many other challenges that startups face. I hope you get to work with such an investor, because it is a true pleasure.
Next, I discuss managing the psychology of the entrepreneurial team, which is of paramount importance in the fickle startup environment.