EDIT: I will note before this, I'm sure others have more experience in scout tanks than I do, but I've spent a fair share of time in them, especially the ELC AMX. So anyone with more experience in other trees or just in general, feel free to correct me, but this is all things I've found to be at least somewhat helpful to the team as a whole while playing bottom/near bottom scout. If you're near the top of the rankings, you might need to play completely differently, but don't forget, even if you're at the top of the rankings, you're still pretty fragile and lots of things can still chew you up. You'll just have a better chance of chewing them up as well.
Scout tanks are fairly hard to play. Unless you wait for the right moment and steal a kill from another tank, it's hard to get kill XP with them, but you've got a few choices to be of benefit to your team.
If you've got a fairly decent gun, you can zip around and harass the enemy, not doing all that much damage, not even really getting that much XP, but distracting the enemy and keeping their big guns focused on little old you and not your bigger teammates which they should be on. Even a bad gun can distract a bad player, as if they're not paying attention, they won't realize they're not actually taking damage from your hits. If your bigger team mates are firing at the same time, they might struggle to figure out where the damaging shots are coming from.
You can literally scout. Some people consider this suicide, and it can be, but ideally you won't rush headlong into the enemy lines and get killed. Learn the maps, learn the paths most people take, learn good places to hide(and TRAIN UP YOUR CAMO SKILL ASAP along with camo netting). You can earn small amounts of spotting xp if you're the first to see the enemy along these paths, and a bit better xp if your team mates attack the enemy outside of their view range, but within yours. Resist the urge to fire unless you're obviously spotted. Unless you've got a big gun, the little bit of damage you can do won't make up for the benefit you're giving with spotting.
You can be sneaky. Try to sneak through the lines. Sometimes this means rushing along a non-standard path, sometimes this means hiding in the bushes while the main line of the enemy passes you by. In any case you can do a lot of damage behind the main line of fighting. Start with their arty. Good arty can win a game, but not if they're dead or distracted by a scout circling and firing on them. Look for enemies that have wedged themselves into places that make it hard for them to turn. On city maps I like to slide right up behind enemies that take shelter in the little destroyed building bunkers and just lay shot after shot into their rear end. Especially effective on TDs or tanks without fully traversing turrets. Use hit and run tactics, don't go for straight out kills if you put yourself at excessive risk. Fire a shot, distract people, give them a reason to turn towards you and not have their strongest armor where it should be, at the main line of battle. And, if the opportunity presents itself, don't give up a chance to just push an enemy off a cliff. Doesn't do much for your score, but it's SO satisfying and fun. Besides, there's usually a reason they want to be up high and not at the bottom of the cliff.
The biggest mistake people make, I believe is rushing out. Don't be in a hurry to use your speed and rush ahead of the line of battle. The only exception to this is if you have a VERY good hiding spot picked out and feel like you can get there to wait before the enemy will see you get into position. This is a risky tactic, but can pay off well. Otherwise, wait. Let the battle actually start a bit, see where the enemy is strongest, where your people are strongest, and find the proper place to push through and do whatever it is you intend to do.
There is one final, somewhat non-standard option. And you might get yelled at by your team mates if you do it, but it's still a perfectly legitimate tactic. Defense. At some point, if the enemy has fast tanks, they'll likely try to push through and do some of these same tactics to you. Especially if you have arty. You can simply wait behind the lines, and fight their fire with your own. Light tanks can usually take on each other, at the very least.
There's also a lot to be said for a small reserve force that isn't engaged and can respond quickly to reinforce an area that's doing poorly. That could be you, if you're patient.
Except in the cases you get lucky, push through and find their arty quickly before it can do much damage, you're unlikely to be the deciding factor in any battle if you're a scout among heavies, but that doesn't mean you can't be a lot of help. It's generally a thankless job, but not a useless one. It can be very tedious though, given how little you gain score for yourself, and how much you have to depend on generally uncoordinated and sometimes outright terrible team mates.