When you are dealing with potential investors, they will be able to do valuation, I hope, but again, they are looking after their own interests. I assume it is a good idea to have your own numbers in mind, especially if you are a single founder. Some companies are being valuated well even before they go to market with a product, So, how can you check if the numbers are inline? How does one go about getting an independent start-up valuation? How accurate are those valuations? Is it based on science of a gut feeling? Is there a formula to be used?
Valuation is more black art than science but there are generally 3 accepted methods
You can immediately see the problem(s)
So investors avoid the issue (along with the tedious yelling/name-calling) by issuing convertible notes which is basically a loan (at high interest rate) with debt-equity swap at discretion of investors. This gives them a chance to see your performance and leaves the tedious arguments about value to a more mature stage. The years of (painful) startup funding people have come up with rough rules of thumbs like $1M / engineer, or $X per unique visitor or $Y per conversion rate. However, this varies so much across domains that only detailed study might come up with a methodology which is acceptable accounting-wise. If you're fortunate enough to be at that stage to pay for such expertise, congratulate yourselves on a value-creation joy-ride.
The Valuation of an early stage startup is basically a totally made up numbers pulled out of thin air (they probably have formulas and Excel sheets designed by very smart people but all the inputs to those formulas are guesses and estimates).
You are giving up a part of the company in exchange for some money, the valuation is used to decide how much of the company you are giving them, they say "your company is worth $X therefor we get Y% for $Z" - but since X is a total made up number you actually have "I've selling Y% of the company for $Z", so you should derive the valuation you are willing to accept from the amount of money you get and the percentage of the company you are willing to sell.
It's just like selling or buying any item that doesn't have an agreed upon price (and it is, you are basically selling a part of your company):
Also, remember this deal isn't dependent on a single number, every clause in every contract and term sheet can (and should be) negotiated.