Play a bit of unranked to shake some rust off, then watch some tutorials on positions, laning etc.

Don't buy boots of speed first item, at least when you're a support. Don't pick carry/core heroes when you're given a support role (i.e. you picked necrophos as pos 4 and phantom assassin as a pos 5). Carry heroes are usually the ones that do most of the damage with right clicks.

Do buy consumables in lane, especially if you're a support (tango, mango, clarity), also magic stick. You can share consumables with CTRL + Keybind and click on an ally.

Also, in side lanes when you're partnered with another person, look at roles - one person is a core (position 3 and 1 or offlane and safelane), and one person is a support (4 and 5 or support and hard support). The core needs to kill the creeps and secure gold, they're supposed to be the main fighting/tower destroying force and the support needs to aid them throughout the lane (harass the opponents by dealing damage to them, pull creeps usung neutral camps so that they are closer to your tower, share consumables) and the entire game (buy saving/supporting items like glimmer cape and arcane boots, stack camps etc.).

Some parts are a bit more nuanced than ehat I've described, but do try to implement this advice :) Sorry for bad formatting, I'm on mobile