There are several ways to get investment into a business. Investors are generally only there for what they can get out of it; the best are ones with business experience who will give advice etc and genuinely help make the business grow, other ones can be like leeches
The key question is "what do they want out of it"
If it is a stake in the company, then control is an issue - you don't want to give too much away. Investors are often only in for a limited time too, so you have to prepare for the time they will want out; if you don't prepare then you might have to sell the co to get the money to buy them back out (bearing in mind the company, and their stake, will have probably gone up in value)
If they are loaning the money, then that is more dodgy as unless you have a proper contract with the length and terms of the loan on it they can effectively pull that loan at any time, leaving you screwed; it's almost always better to go to the bank for investment money and take advantage of any grants going in this scenario as the bank will act less like a loan shark
The background of the investors will say a lot, and as TotallyAwesome says getting written agreements is absolutely key
Keeping proper records will be important if you get an investor on board, not just for taxes but because you are then playing with other peoples' money as well as your own, so your shit needs to be watertight. I've made an easy system that does all this for you incidentally - I've made it for people like plumbers etc etc who want to work for themselves but don't have the office skills - something like that could cut out a lot of the paperwork, which is everyones least favourite part of runnign a business (aside from paying taxes)