This guide is not going to make you rich i can assure you, but it will help you with a systematic approach to getting off the ground.
My background- Built anonymous gym brand of clothing from Gildan tees to cut and and sew polyester fitted garments. Sold the brand at the end of 2015 to begin a new venture. In todays economy, with young and hungry entrepreneurs wanting to avoid the supermarket 9-5. The t shirt market is being flooded. Here are a few of my tips in order to push you in the right direction.

Once you have a viable design and a working website. These are some free ways to promote and drive traffic to your site.

Basic Social Media:
-Being consistent with social media is one of the only options for someone with little to no budget. I would STRONGLY advise keeping it consistent, and very regular. If you can manage 1-2 posts a day for a month you'll see results i promise. Do some market research for the most efficient hashtags to use on each platform to maximise impressions.
-Youtube has effectively the highest conversion rate in all cases when comparing the number of legitimate followers to a conversion. So seek sponsors or shoutouts from Youtube. But don't forget other platforms too, Youtube is just in it's own league.
-Blog posting is great for search engine traffic. Answer questions before they can be asked. This will save you work in the future with FAQ's and also help generate traffic.

Mutual Interest "Sponging":
-Collaborations with other small brands are an opportunity to "sponge" off of one another limited but vital traffic. 
-Backlinks are a segment in SEO that provides traffic from random locations that your link can be found in the internet. So creating an affiliate page on your site offers a reason for people to return the favour.

The Grunt Work:
-There are numerous events in the UK for small brands to gain awareness. Pop up shops, markets and events. Often a stall is relatively cheap at certain events. A UK example is "Crepe City" for fashion related garments.
-I personally took my gym brand to as many gyms as possible, found out about local competitions and powerlifting events.
-Throw some site advertisement leaflets up on gym or small store pinboards, they have been there and are usually more than open to help out a budding entrepreneur.

-Get the google analytics on
-Review conversion rates and which methods of marketing are helping you most, this will help you with time management. 

My general tip to you would be TO NOT ignore the "grunt" or "leg" work. It's the laborious jobs that take time out of your day to complete that tend to work. Consider marketing a numbers game, at some point there is a conversion ration between sales and the number of people that view your product. Find it and strive to reach that number of impressions every day, then every 12 hours, then every hour etc.

Example Day Plan
1. Post On Instagram, be sure to hashtag.
2. Post On Facebook, Be Sure To Hashtag
3. Post On Twitter, Be Sure To Hashtag
4. Post On Pinterest, Be Sure To Hashtag
5. Plan or write a blog post
6. Follow, like and comment on all social media platforms
7. Research your competition (imperative, sometimes it's even good to analyse and copy their strategies)
8. Research market stall opportunities
9. Email some collaboration opportunities
10. Email some Youtubers
11. Weekend work is the grunt work opportunity
More edits to follow.