Usually I find myself with a lot deck ideas and my only challenge is finding the actual cards. But to help you out I'll teach you my method of deck building;
1) Choose a plane in the multiverse to focus on. This will kinda help give your deck a flavor while at the same time giving your deck's overall mechanic a cohesiveness.
2) Choose a theme or concept. You don't have to be a pro deck builder/player to be good at magic, your creativity is usually the "X-Factor" in most duels. Using cards people are familiar with but in unexpected fashions are more often than not the keys to a decisive victory. For example, a lot of people like playing Mono-Green and R/G Hydras, but how often do people build G/U Hydras or Temur (G/U/R) Hydras? Once you choose a concept or style to play your deck in look at ALL the cards from that set to see what would actually make your deck work. This is why I use the Gatherer card search engine on magic the gathering's main page.
3) Choose your colors. Now that you've chosen the world and chosen your theme it's not time to choose your colors. Next to the overall concept of the deck the colors can usually make or break a good deck. However, if you choose correctly and use what you have at your disposal then you'll have a
dangerous deck. For example; most people familiar with Innistrad might build a B/R or W/B vampire deck which all play pretty well. However one thing someone might NOT have tried is playing B/G/U or R/B/G vampires using cards from just that plane.
When I say to focus on a specific plane I'm not telling you to always
exclude outside cards. I say to focus on primarily one world because different card blocks and sets have their own designed game mechanics and it is easier to pick up on this if your deck is nearly all of those cards. Also it's a great way to give your deck a sturdy foundation BEFORE adding new or different cards to your deck. For instance;
Korozda Gorgon is from Return to Ravnica and
Howlgeist is from Innistrad. You probably wouldn't see these two together anytime soon but their synergy does
serve a purpose. The gorgon eats +1/+1 counters and the wolf can constantly come back from the grave because it has no +1/+1 counter. It's small strategies such as this that are able to make or break decks. I hope that my words of advice can help you on your deck building journey.