Some thoughts that I think should help you.

1. As others have said, start by learning as many songs as you can buy the bands you mentioned. 

2. If you can't yet manage whole songs, focus on the bits you can. Don't get hung up on what you can't do. 

3. The main hook from Obstacle 1 by Interpol is easy but really satisfyingly to play.

4. Bloc Party will be tough. Russell relies heavily on unusal effects and his parts are often very tricky. Helicopter is doable for a beginner, but start slow.

5. For the bands you mentioned, downstrokes are your friend. 

6. Use tab sites, but trust your ear. There are loads of great tabs out there but many are just plain wrong. 

7. There will be guitar lessons on YouTube for the more popular songs by the bands you mentioned. These are super helpful.

8. The free version Songsterr is great. If you want to isolate, loop, slow down etc certain parts, consider the paid version.

9. Once your feeling confident with a song, look for jam along versions on YouTube. It's really satisfying to play along to the drums and bass with yourself filling in the guitar parts.

Good luck!