First things first, your business card must provide the key information of your business, your name, your email, your telephone number and your website. If you feel it is necessary to provide a short description of what your business is all about then remember that cluttering too much onto a small space can be detrimental. One thing I would recommend doing is getting a proper email address for example, email@example.com rather than using hotmail or gmail, using such providers looks unprofessional and could put people off.
Can you read things clearly?
Avoid using text that is too small, the minimum font size I would recommend is 8pt, but also take into account that fonts display differently and some may therefore not be readable at this size. Many printing company's such as LS1 Print and Fileclickprint provide business card design services and I would personally recommend going down this road unless you have some design experience. These company's also carry out free artwork checks on designs to ensure everything is ready for print and their experience means that if there is even the slightest issue they will resolve it before sending your design to print.
A well designed business card could be the difference between you getting a client or not. Again this is why I would reiterate getting someone to design it for you, some places only charge around £40-£50, and you are getting the services of someone who does this on a regular basis, understands design trends and has the skills to use the best quality software for the job. However, take into account that bright colours need to be contrasted well together, there is some software that can do this for you online which I would recommend, and don't underestimate the power of simplicity when used in conjunction with a nice font that reflects your business model.
Branding should be consistent across the board, so if you already have branding for your business you should use the same colours and fonts. If you don't have branding then you are free to do as you wish without any constrains, but again, remember to be careful and pick colours that compliment each other. One thing I would mention is that graphic designers and certain good printing companies have high-end monitors that keep colours true on the screen, this is important as you could end up getting print back whose colours would look very different to what you were expecting. Also make sure you get an artwork check!
If you can afford it, I would highly recommend getting spot UV, this is a type of embossing that actually raises your design upwards off the paper. It has been proven a great technique to engage people with more of their senses and therefore improve their recall of your business. Oh, and it also looks brilliant and reflects well on your business.
Borders and Bleeds
Avoid using borders on your business card design, printing is not 100% free from movement and these movements are more likely to show when there is a border. It is recommended to always use at least 2mm-3mm bleed around the edge of your design where all solid colours and images 'bleed' into. This is so that when your business cards are cut down, the colours go right up to the edge.
The Safe Area
When designing a business card it is important to not have textual information too close to the edge of where the card will be cut, this is where the term 'safe area' comes from.
A business card at 300gsm is going to be far too thin and flimsy, and would not fair well with potential clients. Again, you pay for what you get, and the thicker card does cost more money but does reflect much better on your business.